Lecture 1: Regression, identification, and the OLS estimator

Economics 527: Econometric Methods

Motivation and data

A motivating example: Return to education

• Suppose wages are generated according to: \begin{aligned} \ln \text{Wage} = \beta_1 &+ {\color{blue}\beta_2}\,\text{Education} + \beta_3\,\text{Gender} + \beta_4\,\text{Race} \\ &+ \beta_5\,\text{Urban} + \beta_6\,\text{Experience} + U. \end{aligned}

• We have a sample of workers and, for each worker, we observe Wage, Education, etc.

• U collects everything besides Education, Gender, etc. that affects wages. Ability is first on that list, and it is unobserved (latent).

• The unknown coefficient {\color{blue}\beta_2} measures the return to education, holding everything else fixed.

• Questions/Issues:

  • Why in this equation \beta_2 measures the return to education;
  • How to “learn” \beta_2 from random data;
  • What data randomness means;
  • What has to be assumed about the unobserved U to be able to learn \beta_2.

Levels and logs

• Level-level, y = \beta x: \beta = \frac{dy}{dx}, and the proportional change in y per unit of x is \beta / y, which depends on where y is.

• Log-level, \ln y = \beta x: \beta = \frac{d \ln y}{d x} = \frac{dy/dx}{y}, so \beta is the proportional rate of change of y with respect to x. Multiply by 100 for a percentage.

• Log-log, \ln y = \beta \ln x: \beta = \frac{d \ln y}{d \ln x} = \frac{dy/y}{dx/x}, the elasticity of y with respect to x.

Logs often “work better”

• Wages are bounded below by zero and have a long right tail. Their log is close to symmetric.

  

• A symmetric dependent variable is not required by anything that follows, but specifications with logs often “work better” in practice.

Data

• We observe a sample of n workers: \{(Y_i, X_i) : i = 1, \ldots, n\}.

• For worker i, Y_i = \ln \text{Wage}_i is a scalar (1\times 1) and X_i stacks the k regressors, with a 1 in the first position for the intercept: X_i = \begin{pmatrix} 1 \\ \text{Educ}_i \\ \text{Gender}_i \\ \vdots \end{pmatrix} \quad (k \times 1), \qquad \beta = \begin{pmatrix} \beta_1 \\ \beta_2 \\ \vdots \\ \beta_k \end{pmatrix} \quad (k \times 1).

• The wage equation for worker i is Y_i = X_i^\top \beta + U_i = \sum_{j=1}^{k} X_{ij}\,\beta_j + U_i, where X_i^\top \beta = \beta^\top X_i is a scalar.

What is random and what is fixed

• \beta is fixed and unknown.

• X_i, U_i, and therefore Y_i are random.

• The population is the distribution that generates \{(Y_i, X_i) : i = 1, \ldots, n\}, a probability distribution rather than a group of people.

• In a cross-section, each observation is a different worker, so we take the observations to be drawn independently and identically distributed: (Y_i, X_i) \text{ are iid, with joint distribution } F_{Y, X_1, \ldots, X_k}.

• Note that independence is between (Y_i,X_i) and (Y_j,X_j) for i \neq j, not between Y_i and X_i.

• Under the iid assumption, we can take the population to be the joint distribution of one observation (Y_i, X_i).

Modelling the randomness

• A random experiment has a sample space \Omega of outcomes \omega. These are possible states of the world.

• Let X denote education and V ability.

• The pair (X, V) is a function from \Omega to \mathbb{R}^2: for the state of the world \omega, X(\omega) is the education and V(\omega) is the ability.

  

• A probability \mathrm{P} assigns to each event A a number \mathrm{P}(A) \in [0,1]. It induces a distribution for (X, V).

• The joint CDF of (X,V) is a statement about sets of outcomes: for all x, v \in \mathbb{R}, \begin{aligned} F_{X,V}(x,v) &= \mathrm{P}(X \le x,\, V \le v) \\ &= \mathrm{P}\big(\{\omega \in \Omega : X(\omega) \le x,\ V(\omega) \le v\}\big). \end{aligned}

Probability

Random variables and the CDF

• A random variable is a map Y : \Omega \to \mathbb{R}. For a set A \subset \mathbb{R}, \mathrm{P}(Y \in A) = \mathrm{P}\big(\{\omega : Y(\omega) \in A\}\big).

• Taking A = (-\infty, y] gives the cumulative distribution function: F_Y(y) = \mathrm{P}(Y \le y), \qquad y \in \mathbb{R}.

• Properties:

  • F_Y is nondecreasing and right-continuous, F_Y(y) \to 0 as y \to -\infty and F_Y(y) \to 1 as y \to \infty;
  • \mathrm{P}(a < Y \le b) = F_Y(b) - F_Y(a);
  • a jump of F_Y at y_1 has size \mathrm{P}(Y = y_1); where F_Y is continuous, \mathrm{P}(Y = y_1) = 0.

Continuous random variables and the PDF

• When F_Y is differentiable, the probability density function is f_Y(y) = \frac{d F_Y(y)}{dy}, \qquad\text{so}\qquad F_Y(y) = \int_{-\infty}^{y} f_Y(v)\, dv.

• f_Y(y) \ge 0, \displaystyle\int_{-\infty}^{\infty} f_Y(y)\, dy = 1, and for A \subset \mathbb{R}, \mathrm{P}(Y \in A) = \int_A f_Y(y)\, dy.

• A density is not a probability: \mathrm{P}(Y = y) = 0 for every y, and f_Y(y) can exceed one.

• Everything in this course is written for the continuous case. The discrete case replaces integrals with sums.

Expectation

• For a function h, \mathrm{E}\left[h(Y)\right] = \int_{-\infty}^{\infty} h(y)\, f_Y(y)\, dy.

• Throughout, every expectation we write down is assumed to exist and to be finite.

• \mathrm{E}\left[Y\right] is the first moment, \mathrm{E}\left[Y^2\right] the second, and \mathrm{Var}\left(Y\right) = \mathrm{E}\left[(Y - \mathrm{E}\left[Y\right])^2\right] = \mathrm{E}\left[Y^2\right] - (\mathrm{E}\left[Y\right])^2.

• The mean is the best constant predictor of Y under squared loss: \mathrm{E}\left[(Y - c)^2\right] = \mathrm{E}\left[(Y - \mathrm{E}\left[Y\right])^2\right] + (\mathrm{E}\left[Y\right] - c)^2 \ge \mathrm{Var}\left(Y\right), with equality only at c = \mathrm{E}\left[Y\right]. (The cross term 2(\mathrm{E}\left[Y\right] - c)\,\mathrm{E}\left[Y - \mathrm{E}\left[Y\right]\right] is zero.)

• Under absolute loss the minimiser of \mathrm{E}\left[|Y - c|\right] is the median instead. Which loss we use decides which feature of the distribution we end up estimating.

Joint distributions

• For two random variables Y and X on the same \Omega: \begin{aligned} F_{Y,X}(y, x) &= \mathrm{P}(Y \le y,\, X \le x),\\ f_{Y,X}(y, x) &= \frac{\partial^2 F_{Y,X}(y,x)}{\partial y\, \partial x}. \end{aligned}

• f_{Y,X} \ge 0, \displaystyle\int\!\!\int f_{Y,X}(y,x)\, dy\, dx = 1, and for A \subset \mathbb{R}^2, \mathrm{P}\big((Y,X) \in A\big) = \int\!\!\int_A f_{Y,X}(y,x)\, dy\, dx.

• With k regressors the joint CDF of one observation is \begin{aligned} F_{Y, X_1, \ldots, X_k}(y, x_1, \ldots, x_k) &= \mathrm{P}(Y \le y,\ X_1 \le x_1,\ \ldots,\ X_k \le x_k). \end{aligned}

From the joint density to a marginal

• Claim. The marginal density of X is obtained by integrating Y out: f_X(x) = \int_{-\infty}^{\infty} f_{Y,X}(y, x)\, dy.

• Proof. Start from the marginal CDF and differentiate: \begin{aligned} F_X(x) &= \mathrm{P}(X \le x) = \mathrm{P}(X \le x,\ Y < \infty) \\ &= \int_{-\infty}^{x} \int_{-\infty}^{\infty} f_{Y,X}(y, u)\, dy\, du, \\ f_X(x) &= \frac{d F_X(x)}{dx} = \frac{d}{dx}\left[ \int_{-\infty}^{x} \int_{-\infty}^{\infty} f_{Y,X}(y, u)\, dy\, du \right] \\ &= \int_{-\infty}^{\infty} f_{Y,X}(y, x)\, dy, \end{aligned} using \frac{d}{dx}\int_{-\infty}^{x} h(u)\, du = h(x).

• The converse fails: knowing f_X and f_Y does not determine f_{Y,X}. The marginals say nothing about how Y and X move together.

Conditional density

• The density of Y given X = x, defined where f_X(x) > 0: f_{Y \mid X}(y \mid x) = \frac{f_{Y,X}(y, x)}{f_X(x)}, \qquad y \in \mathbb{R}.

• This has the same shape as conditional probability for events, \mathrm{P}(A \mid B) = \mathrm{P}(A \cap B)/\mathrm{P}(B): the joint, rescaled by the marginal density of what we condition on.

• It is a proper density in y: \int_{-\infty}^{\infty} f_{Y \mid X}(y \mid x)\, dy = \frac{1}{f_X(x)} \underbrace{\int_{-\infty}^{\infty} f_{Y,X}(y, x)\, dy}_{=\, f_X(x)} = 1.

Independence

• Y and X are independent if conditioning changes nothing: f_{Y \mid X}(y \mid x) = f_Y(y) \quad \text{for all } y \text{ and all } x \text{ with } f_X(x) > 0.

• Equivalently the joint factorises: \begin{aligned} f_{Y,X}(y, x) &= f_Y(y)\, f_X(x) \quad \text{for all } x, y, \\ \text{or equivalently}\qquad F_{Y,X}(y, x) &= F_Y(y)\, F_X(x) \quad \text{for all } x, y. \end{aligned}

• Independence is a strong statement: it involves the whole conditional distribution. Regression will need much less.

Expectation of a function of two variables

• For a function h of both arguments, \mathrm{E}\left[h(Y, X)\right] = \int_{-\infty}^{\infty}\!\!\int_{-\infty}^{\infty} h(y, x)\, f_{Y,X}(y, x)\, dy\, dx.

• With h(y, x) = yx this is the joint moment \mathrm{E}\left[YX\right]. Centring both arguments gives the covariance: \begin{aligned} \mathrm{Cov}\left(Y, X\right) &= \mathrm{E}\left[\big(Y - \mathrm{E}\left[Y\right]\big)\big(X - \mathrm{E}\left[X\right]\big)\right] \\ &= \mathrm{E}\left[YX\right] - \mathrm{E}\left[Y\right]\,\mathrm{E}\left[X\right]. \end{aligned}

• When \mathrm{Var}\left(Y\right) and \mathrm{Var}\left(X\right) are positive and finite, dividing by the two standard deviations removes the units and gives the correlation, \mathrm{Corr}(Y, X) = \frac{\mathrm{Cov}\left(Y, X\right)}{\sqrt{\mathrm{Var}\left(Y\right)\,\mathrm{Var}\left(X\right)}} \in [-1, 1], where the bound is the Cauchy-Schwarz inequality.

• Y and X are uncorrelated if \mathrm{Cov}\left(Y, X\right) = 0, equivalently \mathrm{E}\left[YX\right] = \mathrm{E}\left[Y\right]\,\mathrm{E}\left[X\right].

• Uncorrelatedness is a single restriction. It says the best straight-line predictor of Y from X under squared loss has zero slope, and it says nothing about any other way the two variables might be related.

Some implications

• Under independence the expectation of a product of separate functions factorises: \begin{aligned} \mathrm{E}\left[h_1(Y)\, h_2(X)\right] &= \int\!\!\int h_1(y)\, h_2(x)\, \underbrace{f_{Y,X}(y, x)}_{=\, f_Y(y)\, f_X(x)}\, dy\, dx \\[2pt] &= \int h_1(y)\, f_Y(y)\, dy \cdot \int h_2(x)\, f_X(x)\, dx \\[2pt] &= \mathrm{E}\left[h_1(Y)\right]\, \mathrm{E}\left[h_2(X)\right]. \end{aligned}

• This holds for every pair of functions h_1 and h_2, not just for the identity.

• Taking h_1(y) = y and h_2(x) = x gives \mathrm{E}\left[YX\right] = \mathrm{E}\left[Y\right]\mathrm{E}\left[X\right], so \mathrm{Cov}\left(Y, X\right) = 0: independence implies uncorrelatedness.

• The converse fails, and it fails for the reason above: uncorrelatedness is the factorisation for one pair of functions, whereas independence is the factorisation for all of them.

Conditional expectation

Conditional expectation: at a point

• Fix x with f_X(x) > 0 and take the mean of the conditional density: \mathrm{E}\left[Y \mid X = x\right] = \int_{-\infty}^{\infty} y\, f_{Y \mid X}(y \mid x)\, dy. This is a number.

• Repeat for every x with f_X(x) > 0. The result is a function of x, x \longmapsto \mathrm{E}\left[Y \mid X = x\right] =: g(x), the conditional expectation function.

Conditional expectation: as a random variable

• Now plug the random variable X into g: \mathrm{E}\left[Y \mid X\right] := g(X) = \int_{-\infty}^{\infty} y\, f_{Y \mid X}(y \mid X)\, dy. This is a random variable: a different draw of X gives a different value.

• The distinction matters for every proof that follows. \mathrm{E}\left[Y \mid X = x\right] is a number indexed by x; \mathrm{E}\left[Y \mid X\right] is the random variable obtained by evaluating that function at X.

• The same construction applied to the squared deviation gives the conditional variance, \mathrm{Var}\left(Y \mid X\right) = \mathrm{E}\left[\big(Y - \mathrm{E}\left[Y \mid X\right]\big)^2 \,\Big|\, X\right], again a random variable, and a function of X.

• Since \mathrm{E}\left[Y \mid X\right] is random, it has a mean of its own. What is it?

Law of iterated expectations

• Claim (LIE). \mathrm{E}\left[\mathrm{E}\left[Y \mid X\right]\right] = \mathrm{E}\left[Y\right].

• Averaging the conditional mean over the distribution of X gives back the unconditional mean.

• Read from right to left, it says any expectation can be computed in two stages: first conditional on X, then over X. Most proofs in this course use it that way.

Law of iterated expectations: proof

• Write the outer expectation as an integral against f_X, then unpack g: \begin{aligned} \mathrm{E}\left[\mathrm{E}\left[Y \mid X\right]\right] &= \int_{-\infty}^{\infty} \underbrace{\mathrm{E}\left[Y \mid X = x\right]}_{g(x)}\, f_X(x)\, dx \\ &= \int_{-\infty}^{\infty} \left[ \int_{-\infty}^{\infty} y\, f_{Y \mid X}(y \mid x)\, dy \right] f_X(x)\, dx \\ &= \int_{-\infty}^{\infty} \int_{-\infty}^{\infty} y\, \underbrace{f_{Y \mid X}(y \mid x)\, f_X(x)}_{=\, f_{Y,X}(y,x)}\, dy\, dx \\ &= \int_{-\infty}^{\infty} y \underbrace{\left[ \int_{-\infty}^{\infty} f_{Y,X}(y, x)\, dx \right]}_{=\, f_Y(y)} dy \\ &= \int_{-\infty}^{\infty} y\, f_Y(y)\, dy = \mathrm{E}\left[Y\right]. \end{aligned}

• The step from the third line to the fourth changes the order of integration.

Taking out what is known

• Conditional on X, any function of X is a constant and comes out of the expectation: \begin{aligned} \mathrm{E}\left[Y \cdot h(X) \mid X\right] &= \int_{-\infty}^{\infty} y\, h(X)\, f_{Y \mid X}(y \mid X)\, dy \\ &= h(X) \int_{-\infty}^{\infty} y\, f_{Y \mid X}(y \mid X)\, dy = h(X)\, \mathrm{E}\left[Y \mid X\right]. \end{aligned}

• Two special cases used constantly: \mathrm{E}\left[h(X) \mid X\right] = h(X), \qquad \mathrm{E}\left[c \mid X\right] = c.

Question

• Let Y, X, Z be random variables. Which of the following equals \mathrm{E}\left[\mathrm{E}\left[Y \mid X, Z\right] \mid Z\right]?
  1. \mathrm{E}\left[Y\right]
  2. \mathrm{E}\left[Y \mid X\right]
  3. \mathrm{E}\left[Y \mid Z\right]
  4. 0

Mean independence

• Y is mean independent of X if the conditional mean does not move with the conditioning value: \mathrm{E}\left[Y \mid X = x\right] = c \quad \text{for all } x \text{ with } f_X(x) > 0, where the constant c does not depend on x.

• Question. The constant c does not depend on x. What must it be?

  1. 0
  2. \mathrm{E}\left[X\right]
  3. \mathrm{E}\left[Y\right]
  4. 42

• Mean independence is weaker than independence:

  • Under mean independence, \mathrm{E}\left[Y \mid X\right] does not depend on X, but \mathrm{E}\left[Y^2\mid X\right], \mathrm{Var}\left(Y \mid X\right), etc., can change with X.
  • Under independence, the whole conditional distribution is constant in X, so \mathrm{Var}\left(Y \mid X\right), etc., are constant too.

A zero conditional mean

• Suppose \mathrm{E}\left[U \mid X\right] = 0 . By the previous slide this is mean independence of U from X, in the case where the constant is zero.

• Two consequences, by the LIE and by taking out what is known: \begin{aligned} \mathrm{E}\left[U\right] &= \mathrm{E}\left[\underbrace{\mathrm{E}\left[U \mid X\right]}_{=\,0}\right] = 0, \\[2pt] \mathrm{E}\left[U X\right] &= \mathrm{E}\left[\mathrm{E}\left[U X \mid X\right]\right] = \mathrm{E}\left[X \underbrace{\mathrm{E}\left[U \mid X\right]}_{=\,0}\right] = 0. \end{aligned} The second line takes X out of the inner expectation, which is allowed because X is known once we condition on it.

• Putting the two together, \mathrm{Cov}\left(U, X\right) = \mathrm{E}\left[UX\right] - \mathrm{E}\left[U\right]\,\mathrm{E}\left[X\right] = 0 - 0 = 0 , so U and X are uncorrelated.

Three concepts of unrelatedness

• Ordered from strongest to weakest: \text{independence} \;\Longrightarrow\; \mathrm{E}\left[U \mid X\right] = \mathrm{E}\left[U\right] \;\Longrightarrow\; \mathrm{Cov}\left(U, X\right) = 0, and neither arrow reverses.

• Mean independence allows \mathrm{Var}\left(U \mid X\right) to depend on X; independence does not. A variable can have a conditional mean that never moves with X while its conditional variance does.

• Uncorrelatedness is weaker still: \mathrm{Cov}\left(U, X\right) = 0 is a single restriction, and it is compatible with a conditional mean \mathrm{E}\left[U \mid X\right] that varies with X, because \mathrm{Cov}\left(U, X\right) = \mathrm{Cov}\left(\mathrm{E}\left[U \mid X\right], X\right).

Conditional mean minimises mean squared error

• Among all functions m of X, the conditional mean is the best predictor of Y under squared loss. Condition on X first: \begin{aligned} \mathrm{E}\left[(Y - m(X))^2 \mid X\right] &= \mathrm{E}\left[\big(Y - \mathrm{E}\left[Y \mid X\right]\big)^2 \mid X\right] \\ &\quad + \big(\mathrm{E}\left[Y \mid X\right] - m(X)\big)^2, \end{aligned} because the cross term is 2\,(\mathrm{E}\left[Y\mid X\right] - m(X))\,\mathrm{E}\left[Y - \mathrm{E}\left[Y \mid X\right] \mid X\right] = 0.

• Now average over X with the LIE: \mathrm{E}\left[(Y - m(X))^2\right] \ge \mathrm{E}\left[\big(Y - \mathrm{E}\left[Y \mid X\right]\big)^2\right], with equality only when m(X) = \mathrm{E}\left[Y \mid X\right] with probability one.

• This is the reason regression is about \mathrm{E}\left[Y \mid X\right] and not some other feature of the conditional distribution. Change the loss and a different feature wins.

Linear regression model

Object of interest

• The relationship between the regressors and the dependent variable is summarised by the conditional expectation function: \mathrm{E}\left[Y_i \mid X_i\right] = \mathrm{E}\left[Y_i \mid X_{i1}, \ldots, X_{ik}\right], for instance \mathrm{E}\left[\ln \text{Wage}_i \mid \text{Education}_i, \text{Experience}_i, \text{Race}_i, \text{Gender}_i, \ldots\right].

• The practical problem: with data \{(Y_i, X_i) : i = 1, \ldots, n\}, estimate \mathrm{E}\left[Y_i \mid X_i\right].

• Without restrictions this is a function of k arguments to be learned from n points. The parametric approach assumes the functional form and leaves a finite number of unknown constants.

Linear regression model

• Assumption. The conditional expectation is linear in the parameters: \mathrm{E}\left[Y_i \mid X_i\right] = \beta_1 X_{i1} + \cdots + \beta_k X_{ik} = X_i^\top \beta, where X_i = (X_{i1}, \ldots, X_{ik})^\top and \beta = (\beta_1, \ldots, \beta_k)^\top \in \mathbb{R}^k.

• Write \mathrm{E}\left[Y_i \mid X_i = x\right] = x^\top \beta = \sum_{l=1}^{k} \beta_l x_l. Differentiating in x_j, \frac{\partial \mathrm{E}\left[Y_i \mid X_i = x\right]}{\partial x_j} = \frac{\partial (\beta_1 x_1 + \cdots + \beta_k x_k)}{\partial x_j} = \beta_j .

• So \beta_j is the change in the conditional mean of Y_i per unit change in X_{ij}, holding the other regressors fixed: a marginal effect.

• “Linear” refers to \beta. The regressors can be anything: squares, logs, products, indicator variables.

Error term

• Define the error as the deviation from the conditional mean: U_i := Y_i - \mathrm{E}\left[Y_i \mid X_i\right] = Y_i - X_i^\top \beta.

• Then, by construction, \begin{aligned} Y_i &= X_i^\top \beta + U_i, \\ \mathrm{E}\left[U_i \mid X_i\right] &= \underbrace{\mathrm{E}\left[Y_i \mid X_i\right]}_{=\, X_i^\top\beta} - \underbrace{\mathrm{E}\left[X_i^\top \beta \mid X_i\right]}_{=\, X_i^\top\beta} = 0. \end{aligned}

• U_i is not observable: the conditional expectation function is unknown, so the deviation from it is unknown.

• If the pairs (Y_i, X_i) are iid, then so are the U_i, since each is the same function of its own pair.

Two ways to write the same model

• Either start from the conditional mean and define the error, \left.\begin{aligned} \mathrm{E}\left[Y_i \mid X_i\right] &= X_i^\top \beta \\ U_i &:= Y_i - X_i^\top \beta \end{aligned}\right\} \quad\Longrightarrow\quad \mathrm{E}\left[U_i \mid X_i\right] = 0,

• or start from the equation and the restriction on the error, \left.\begin{aligned} Y_i &= X_i^\top \beta + U_i \\ \mathrm{E}\left[U_i \mid X_i\right] &= 0 \end{aligned}\right\} \quad\Longrightarrow\quad \mathrm{E}\left[Y_i \mid X_i\right] = X_i^\top \beta.

Intercept

• Consider an example with a single regressor X_i^\ast\in \mathbb{R}.

• Suppose Y_i = \beta^\ast X_i^\ast + U_i^\ast with \mathrm{E}\left[U_i^\ast\mid X_i^\ast\right] = \alpha \neq 0.

• We can add and subtract \alpha to get: Y_i = \alpha + \beta^\ast X_i^\ast + \underbrace{(U_i^\ast - \alpha)}_{U_i}, \qquad \mathrm{E}\left[U_i\mid X_i^\ast\right] = 0.

• Define vectors X_i= (1, X_i^\ast)^\top and \beta = (\alpha, \beta^\ast)^\top. Then Y_i = X_i^{\top} \beta + U_i, \qquad \mathrm{E}\left[U_i\mid X_i\right] = \mathrm{E}\left[U_i\mid 1, X_i^\ast\right]= 0, as conditioning on 1 in addition to X_i^\ast does not change the conditional expectation.

• Here \alpha is the intercept: a coefficient on a constant regressor (i.e., a regressor equal to 1 for all observations i).

• We include the intercept to absorb constant but non-zero conditional means of the error. This does not change the slope coefficients.

• However, if the conditional mean of the error is not constant, i.e., \mathrm{E}\left[U_i^\ast\mid X_i^\ast\right] = \alpha+\gamma X_i^\ast, then Y_i = \alpha + (\beta^\ast+\gamma) X_i^\ast + \underbrace{(U_i^\ast - \alpha-\gamma X_i^\ast)}_{U_i}, \qquad \mathrm{E}\left[U_i\mid X_i^\ast\right] = 0, and the slope coefficient is affected.

Identification

Identification: the question

• Consider the model Y_i = X_i^\top \beta + U_i with \mathrm{E}\left[U_i \mid X_i\right] = 0:

  • Y_i,X_i are observed,
  • U_i is unobserved,
  • \beta\in\mathbb R^k is unknown.

• Suppose the joint distribution of the observables F_{Y, X_1, \ldots, X_k}(y, x_1, \ldots, x_k) is known (we are at the population level). You have no information about U_i beyond \mathrm{E}\left[U_i \mid X_i\right] = 0.

• Can you recover \beta?

• If the answer is no with the whole distribution in hand, no amount of data will help, as the joint distribution of the data does not pin down \beta.

Identification: the definition

• Definition. \beta is identified if it is uniquely determined by the joint distribution of the observed variables (Y_i, X_{i1}, \ldots, X_{ik}).

• Two things can go wrong:

  • the distribution of the observables is consistent with more than one value of \beta;
  • it is consistent with none, because the model is wrong.

• We ask: what does \mathrm{E}\left[U_i \mid X_i\right] = 0 let us compute from the distribution of (Y_i, X_i), and does it pin \beta down?

What the model gives us

• By \mathrm{E}\left[U_i \mid X_i\right] = 0 and the LIE, \mathrm{E}\left[X_i U_i\right]=0.

• Rewrite U_i in terms of the observables and \beta: U_i = Y_i - X_i^\top \beta

• Obtain the population normal equations: \boxed{\ 0 = \mathrm{E}\left[X_i U_i\right] = \mathrm{E}\left[X_i \big(Y_i - X_i^\top \beta\big)\right].\ }

• Multiply out, one step at a time, distribute the expectation: \begin{aligned} 0 = \mathrm{E}\left[X_i U_i\right] &= \mathrm{E}\left[X_i \big(Y_i - X_i^\top \beta\big)\right] \\ &= \mathrm{E}\left[X_i Y_i - X_i X_i^\top \beta\right] \\ &= \mathrm{E}\left[X_i Y_i\right] - \mathrm{E}\left[X_i X_i^\top \beta\right] \\ &= \mathrm{E}\left[X_i Y_i\right] - \mathrm{E}\left[X_i X_i^\top\right]\,\beta , \end{aligned} the last step because \beta is a vector of constants and comes out of the expectation.

• Rearranging, \mathrm{E}\left[X_i X_i^\top\right]\,\beta = \mathrm{E}\left[X_i Y_i\right].

• Knowns: \mathrm{E}\left[X_i Y_i\right] (k\times1) and \mathrm{E}\left[X_i X_i^\top\right] (k \times k), both features of the distribution of the observables. Unknown: \beta.

• If \mathrm{E}\left[X_i X_i^\top\right] is invertible, the system has exactly one solution. Whether it is invertible is the whole question.

Second-moment matrix

• Write X_i as a column and X_i^\top as a row, and multiply: \begin{aligned} \underset{k\times1}{X_i}\ \underset{1\times k}{X_i^\top} &= \begin{pmatrix} X_{i1} \\ X_{i2} \\ \vdots \\ X_{ik} \end{pmatrix} \begin{pmatrix} X_{i1} & X_{i2} & \cdots & X_{ik} \end{pmatrix} \\[4pt] &= \begin{pmatrix} X_{i1}^2 & X_{i1} X_{i2} & \cdots & X_{i1} X_{ik} \\ X_{i2} X_{i1} & X_{i2}^2 & \cdots & X_{i2} X_{ik} \\ \vdots & \vdots & \ddots & \vdots \\ X_{ik} X_{i1} & X_{ik} X_{i2} & \cdots & X_{ik}^2 \end{pmatrix} \quad (k \times k). \end{aligned} Entry (j,l) of the product is X_{ij} X_{il}.

• The expectation is taken entry by entry: \underset{k\times k}{\mathrm{E}\left[X_i X_i^\top\right]} = \mathrm{E}\begin{bmatrix} X_{i1}^2 & X_{i1} X_{i2} & \cdots & X_{i1} X_{ik} \\ X_{i2} X_{i1} & X_{i2}^2 & \cdots & X_{i2} X_{ik} \\ \vdots & \vdots & \ddots & \vdots \\ X_{ik} X_{i1} & X_{ik} X_{i2} & \cdots & X_{ik}^2 \end{bmatrix}.

• It is symmetric: using (AB)^\top = B^\top A^\top, \big(\mathrm{E}\left[X_i X_i^\top\right]\big)^\top = \mathrm{E}\left[(X_i X_i^\top)^\top\right] = \mathrm{E}\left[X_i X_i^\top\right].

Second-moment matrix is positive semidefinite

• Definitions. A k\times k symmetric matrix A is positive semidefinite (p.s.d.) if c^\top A c \ge 0 for all c \in \mathbb{R}^k, and positive definite (p.d.) if c^\top A c > 0 for all c \neq 0.

• Take any c \in \mathbb{R}^k and define the scalar W_i = X_i^\top c = c_1 X_{i1} + \cdots + c_k X_{ik}, \qquad c^\top X_i = (X_i^\top c)^\top = W_i.

• Then the quadratic form is the expectation of a square: c^\top\, \mathrm{E}\left[X_i X_i^\top\right]\, c = \mathrm{E}\left[\underbrace{c^\top X_i}_{W_i} \underbrace{X_i^\top c}_{{W_i}}\right] = \mathrm{E}\left[\underbrace{W_i^2}_{\ge 0}\right] \ge 0.

• So \mathrm{E}\left[X_i X_i^\top\right] is p.s.d. whatever the distribution of the regressors. Positive definiteness is the extra step, and it can fail.

When positive definiteness fails

• Suppose some \tilde c \neq 0 gives \tilde c^\top \mathrm{E}\left[X_i X_i^\top\right]\, \tilde c = 0. With \tilde W_i = X_i^\top \tilde c, \begin{aligned} 0 = \tilde c^\top \mathrm{E}\left[X_i X_i^\top\right]\, \tilde c = \mathrm{E}\left[\tilde W_i^2\right] &\;\Longleftrightarrow\; \tilde W_i = 0 \text{ with probability } 1 \\ &\;\Longleftrightarrow\; \boxed{\ \tilde c_1 X_{i1} + \cdots + \tilde c_k X_{ik} = 0\ } \end{aligned}

• With \tilde c \ne 0, say \tilde c_1 \neq 0, one regressor equals, with probability one, an exact linear combination of the others: X_{i1} = -\frac{\tilde c_2}{\tilde c_1} X_{i2} - \cdots - \frac{\tilde c_k}{\tilde c_1} X_{ik}. This is perfect multicollinearity.

Positive definiteness and the rank condition

• Putting the two directions together, \begin{aligned} \mathrm{E}\left[X_i X_i^\top\right] \text{ is p.d.} &\;\Longleftrightarrow\; X_i^\top c = 0 \text{ only if } c = 0 \\ &\;\Longleftrightarrow\; \mathrm{E}\left[X_i X_i^\top\right] c=0 \text{ only if } c = 0 \\ &\;\Longleftrightarrow\; \operatorname{rank}\big(\mathrm{E}\left[X_i X_i^\top\right]\big) = k. \end{aligned}

• Ruled out: A relation c_1 X_{i1} + c_2 X_{i2} = 0 for some (c_1, c_2) \neq (0,0).

• X_{i2} = X_{i1}^2 is not linear and is fine, unless X_{i1} is binary, when X_{i1}^2 = X_{i1}.

Identification: the result

• Assume
  1. Y_i = X_i^\top \beta + U_i with \beta \in \mathbb{R}^k;
  2. \mathrm{E}\left[U_i \mid X_i\right] = 0;
  3. No perfect multicollinearity: \operatorname{rank}\big(\mathrm{E}\left[X_i X_i^\top\right]\big) = k.
• Then \beta is identified:
  • Assumption 1 and 2 produce: \mathrm{E}\left[X_i (Y_i - X_i^\top \beta)\right] = 0.
  • This implies \mathrm{E}\left[X_i X_i^\top\right]\beta = \mathrm{E}\left[X_i Y_i\right].
  • Assumption 3 makes \mathrm{E}\left[X_i X_i^\top\right] invertible, so \boxed{\ \beta = \big(\mathrm{E}\left[X_i X_i^\top\right]\big)^{-1}\, \mathrm{E}\left[X_i Y_i\right]\ }

Estimation and least squares

From identification to an estimator

• Identification wrote \beta as a function of population moments: \beta = \big(\mathrm{E}\left[X_i X_i^\top\right]\big)^{-1}\, \mathrm{E}\left[X_i Y_i\right].

• Method of moments. Replace each expectation \mathrm{E}\left[\cdot\right] by the sample average \frac{1}{n}\sum_{i=1}^n (\cdot): \hat\beta = \left( \frac{1}{n}\sum_{i=1}^{n} X_i X_i^\top \right)^{-1} \frac{1}{n}\sum_{i=1}^{n} X_i Y_i = \left( \sum_{i=1}^{n} X_i X_i^\top \right)^{-1} \sum_{i=1}^{n} X_i Y_i. The two factors of \frac{1}{n} cancel.

• An estimator is any function of the sample \{(Y_i, X_i)\}. It may not depend on U_i or on \beta, which are unobserved.

Same estimator from the moment condition

• Alternatively, start from \mathrm{E}\left[X_i (Y_i - X_i^\top \beta)\right] = 0 and replace the expectation by an average, evaluated at the estimator: \frac{1}{n} \sum_{i=1}^{n} X_i \big(\underbrace{Y_i - X_i^\top \hat\beta}_{=:\ \hat U_i}\big) = 0 \quad\Longrightarrow\quad \sum_{i=1}^{n} X_i X_i^\top\, \hat\beta = \sum_{i=1}^{n} X_i Y_i, which is the same \hat\beta.

• \hat U_i = Y_i - X_i^\top \hat\beta is the residual: the sample counterpart of the error U_i = Y_i - X_i^\top \beta. Unlike U_i, it is computable.

• \hat\beta is built to make the residuals satisfy \frac{1}{n} \sum_{i=1}^{n} X_i \hat U_i = 0, which is the sample version of \mathrm{E}\left[X_i U_i\right] = 0.

Estimator is not the parameter

• Sample averages are random; expectations are fixed numbers. In general \frac{1}{n}\sum_{i=1}^{n} X_i X_i^\top \neq \mathrm{E}\left[X_i X_i^\top\right], \qquad \frac{1}{n}\sum_{i=1}^{n} X_i Y_i \neq \mathrm{E}\left[X_i Y_i\right], and so in general \hat\beta \neq \beta.

• Flip a fair coin ten times: the sample proportion of heads is rarely one half, although the population proportion is exactly one half.

• If the errors U_1, \ldots, U_n are continuously distributed, \mathrm{P}(\hat\beta = \beta) = 0.

• \hat\beta is a random vector: a different sample gives a different value. It has a distribution, and the next lecture is about where that distribution sits relative to \beta and how spread out it is.

Sample orthogonality

• Since \hat\beta solves \sum_i X_i (Y_i - X_i^\top \hat\beta) = 0, \sum_{i=1}^{n} X_i \hat U_i = 0 \qquad\Longleftrightarrow\qquad \begin{pmatrix} \sum_{i=1}^{n} X_{i1} \hat U_i \\ \vdots \\ \sum_{i=1}^{n} X_{ik} \hat U_i \end{pmatrix} = \begin{pmatrix} 0 \\ \vdots \\ 0 \end{pmatrix}.

• If the first regressor is the constant, X_{i1} = 1, the first equation says \sum_{i=1}^n \hat U_i = 0: the residuals average to zero.

Matrix notation

• Stack the n observations: \underset{n\times1}{Y} = \begin{pmatrix} Y_1 \\ \vdots \\ Y_n \end{pmatrix},\qquad \underset{n\times1}{U} = \begin{pmatrix} U_1 \\ \vdots \\ U_n \end{pmatrix},\qquad \underset{n\times k}{X} = \begin{pmatrix} X_1^\top \\ \vdots \\ X_n^\top \end{pmatrix}.

• Row i of X is the transpose of the regressor vector X_i; column j of X holds regressor j for all n observations: X = \begin{pmatrix} X_{11} & X_{12} & \cdots & X_{1k} \\ \vdots & \vdots & & \vdots \\ X_{n1} & X_{n2} & \cdots & X_{nk} \end{pmatrix}.

• The n equations Y_i = X_i^\top \beta + U_i become one: Y = \begin{pmatrix} X_1^\top \beta \\ \vdots \\ X_n^\top \beta \end{pmatrix} + \begin{pmatrix} U_1 \\ \vdots \\ U_n \end{pmatrix} \qquad\Longleftrightarrow\qquad \underset{n\times1}{Y} = \underset{n\times k}{X}\ \underset{k\times1}{\beta} + \underset{n\times1}{U}.

Two identities and the matrix form of the estimator

• Claim. X^\top Y = \sum_{i=1}^{n} X_i Y_i \quad (k \times 1), \qquad X^\top X = \sum_{i=1}^{n} X_i X_i^\top \quad (k \times k).

• Proof of the second. Transposing the stack of rows gives a row of columns: \begin{aligned} X^\top X &= \begin{pmatrix} X_1^\top \\ \vdots \\ X_n^\top \end{pmatrix}^{\top} \begin{pmatrix} X_1^\top \\ \vdots \\ X_n^\top \end{pmatrix} = \begin{pmatrix} X_1 & \cdots & X_n \end{pmatrix} \begin{pmatrix} X_1^\top \\ \vdots \\ X_n^\top \end{pmatrix} \\ &= \underbrace{X_1 X_1^\top}_{k\times k} + \cdots + X_n X_n^\top = \sum_{i=1}^{n} X_i X_i^\top. \end{aligned} The first identity is the same computation with the second factor X replaced by Y, whose ith row is the scalar Y_i.

• Substitute the two identities into the method-of-moments formula: \begin{aligned} \underset{k\times1}{\hat\beta} &= \left( \sum_{i=1}^{n} X_i X_i^\top \right)^{-1} \sum_{i=1}^{n} X_i Y_i \\ &= \Big( \underset{k\times k}{X^\top X} \Big)^{-1} \underset{k\times1}{X^\top Y}. \end{aligned}

Residuals in matrix form

• Residual vector and its defining property: \begin{aligned} \hat U &= Y - X \hat\beta, \\ X^\top \hat U &= X^\top (Y - X\hat\beta) = X^\top Y - X^\top X\, \underbrace{(X^\top X)^{-1} X^\top Y}_{\hat\beta} \\ &= X^\top Y - X^\top Y = 0. \end{aligned}

Sample rank condition

• \operatorname{rank}\left(X^\top X\right)= \operatorname{rank}\left(X\right).

• Since X^\top X is k\times k, its inverse exists exactly when \operatorname{rank}\left(X\right) = k, meaning the columns of X are linearly independent.

• Let c \in \mathbb{R}^k and consider the linear combination of the columns of X: X c = c_1 \begin{pmatrix} X_{11} \\ \vdots \\ X_{n1} \end{pmatrix} + \cdots + c_k \begin{pmatrix} X_{1k} \\ \vdots \\ X_{nk} \end{pmatrix} = 0 \quad\text{only if}\quad c = 0, In particular n \ge k: at least as many observations as coefficients.

• What it means observation by observation: c_1 X_{i1} + \cdots + c_k X_{ik} = 0 for all i = 1, \ldots, n only if c = 0.

• But this is equivalent to the population condition \operatorname{rank}(\mathrm{E}\left[X_i X_i^\top\right]) = k, when n\geq k.

Least squares

• Take a candidate value b \in \mathbb{R}^k and measure how badly the fitted values X_i^\top b fit the data by the sum of squared residuals: \begin{aligned} S(b) = \sum_{i=1}^{n} \big(Y_i - X_i^\top b\big)^2 &= \begin{pmatrix} Y_1 - X_1^\top b \\ \vdots \\ Y_n - X_n^\top b \end{pmatrix}^{\top} \begin{pmatrix} Y_1 - X_1^\top b \\ \vdots \\ Y_n - X_n^\top b \end{pmatrix} \\ &= (Y - Xb)^\top (Y - Xb) = \|Y - Xb\|^2, \end{aligned} where for a \in \mathbb{R}^n, \|a\|^2 = a^\top a = \sum_{i=1}^n a_i^2.

• The ordinary least squares estimator picks the b that fits best: \hat\beta^{\,\text{OLS}} = \underset{b \in \mathbb{R}^k}{\arg\min}\ \sum_{i=1}^{n} \big(Y_i - X_i^\top b\big)^2 = \underset{b \in \mathbb{R}^k}{\arg\min}\ \|Y - Xb\|^2.

• Claim. \hat\beta^{\,\text{OLS}} = \hat\beta = (X^\top X)^{-1} X^\top Y: least squares and the method of moments deliver the same estimator.

Proof

• We will use:

  • (u + v)^\top (u + v) = u^\top u + v^\top v + 2 u^\top v.
  • Add and subtract X\hat\beta inside the criterion: Y-Xb=Y-X\hat\beta+X\hat\beta-Xb=\underbrace{Y-X\hat\beta}_{\hat U}+X(\hat\beta-b).

• We have: \begin{aligned} (Y - Xb)^\top (Y - Xb) &= \hat U^\top \hat U + (\hat\beta - b)^\top X^\top X (\hat\beta - b) \\ &\qquad + 2\, {\color{red} {\hat U^\top X}} (\hat\beta - b) \end{aligned}

• The cross term vanishes by the normal equations {\color{red} X^\top \hat U = 0}: \boxed{(Y - Xb)^\top (Y - Xb)= \underbrace{\hat U^\top \hat U}_{\text{free of } b} + (\hat\beta - b)^\top X^\top X (\hat\beta - b).}

• So minimising (Y - Xb)^\top (Y - Xb) is the same as minimising (\hat\beta - b)^\top X^\top X (\hat\beta - b).

• (\hat\beta - b)^\top X^\top X (\hat\beta - b)\ge 0 as this is a quadratic form in X^\top X, which is always p.s.d.

• When \operatorname{rank}\left(X\right)=k, X^\top X is p.d. and (\hat\beta - b)^\top X^\top X (\hat\beta - b) =0 \quad\Longleftrightarrow\quad \hat\beta-b=0.

• Hence, \hat\beta is the unique minimiser of (Y - Xb)^\top (Y - Xb) .

Other linear estimators, for comparison

• Other estimators can be easily constructed.

• For a known symmetric positive definite n \times n matrix W, let \tilde\beta_W = (X^\top W X)^{-1} X^\top W Y

• Note W = I_n gives back \hat\beta.

• With W diagonal, W = \operatorname{diag}(w_1, \ldots, w_n) and w_i > 0, this is weighted least squares: \tilde\beta_W = \left(\sum_{i=1}^{n} w_i X_i X_i^\top\right)^{-1} \sum_{i=1}^{n} w_i X_i Y_i.

• Which of these is a good estimator, and in what sense, is the subject of the next lecture: bias, variance, and the Gauss-Markov theorem.

Summary

Summary: the model and identification

• The object of interest is the conditional expectation \mathrm{E}\left[Y_i \mid X_i\right]; the linear model assumes \mathrm{E}\left[Y_i \mid X_i\right] = X_i^\top \beta, which is the same as Y_i = X_i^\top \beta + U_i with \mathrm{E}\left[U_i \mid X_i\right] = 0.

• Probability supplied what was needed on the way: densities and conditional densities, the law of iterated expectations, taking out what is known, and mean independence.

• Identification. \mathrm{E}\left[U_i \mid X_i\right] = 0 gives \mathrm{E}\left[X_i U_i\right] = 0, hence the population normal equations \mathrm{E}\left[X_i (Y_i - X_i^\top \beta)\right] = 0, that is \mathrm{E}\left[X_i X_i^\top\right]\beta = \mathrm{E}\left[X_i Y_i\right]. If \mathrm{E}\left[X_i X_i^\top\right] is positive definite, which means no exact linear relation among the regressors, then \beta = \big(\mathrm{E}\left[X_i X_i^\top\right]\big)^{-1} \mathrm{E}\left[X_i Y_i\right].

Summary: the estimator

• Estimation. Under the sample rank condition \operatorname{rank}(X) = k, replacing expectations by sample averages gives \hat\beta = \left( \sum_{i=1}^{n} X_i X_i^\top \right)^{-1} \sum_{i=1}^{n} X_i Y_i = (X^\top X)^{-1} X^\top Y, which is also the unique minimiser of \sum_i (Y_i - X_i^\top b)^2: the OLS estimator.

• \hat\beta solves the sample normal equations X^\top (Y - X\hat\beta) = 0, that is X^\top \hat U = 0: the sample counterpart of the population normal equations \mathrm{E}\left[X_i (Y_i - X_i^\top \beta)\right] = 0.

• \hat\beta is also the OLS estimator as it solves \min_b \|Y-Xb\|^2.