Instrumental Variables Regression. One Regressor and One Instrument The General IV Regression Model Checking Instrument Validity Application Where Do Valid Instruments Come From?. Three important threats to internal validity are:
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Instrumental variables regression can eliminate bias when E(u|X) ≠ 0- using an instrumental variable, Z.
An endogenous variable is one that is correlated with u.
An exogenous variable is one that is uncorrelated with u.
For an instrumental variable (an “instrument”) Z to be valid, it must satisfy two conditions:
Suppose for now that you have such a Zi (we’ll discuss how to find instrumental variables later), How can you use Zi to estimate ?
Explanation #1: Two Stage Least Squares (TSLS)
As it sounds, TSLS has two stages - two regressions:
(1)First isolates the part of X that is uncorrelated with u: regress X on Z using OLS.
, i = 1, ... , n.
regress Y on using OLS:
Regress Xi on Zi , obtain the predicted values .
Regress Yi on , the coefficient on is the TSLS estimator, .
Then is a consistent estimator of .
where (instrument exogeneity).
The IV estimator replaces these population covariances with sample covariances.
sYZ and sXZ are the sample covariances
This is the TSLS estimator - just a different derivation.
Derivation of sample covariances.
The sample covariances are consistent:
IV regression was originally developed to estimate demand elasticities for agricultural goods, for example butter:
Simultaneous causality bias in the OLS regression of on arises because price and quantity are determined by the interaction of demand and supply.
This interaction of demand and supply produces on arises because price and quantity are determined by the interaction of demand and supply.
Would a regression using these data produce the demand curve? No!
What would you get if only supply shifted? on arises because price and quantity are determined by the interaction of demand and supply.
TSLS estimates the demand curve by isolating shifts in price and quantity that arise from shifts in supply.
Z is a variable that shifts supply but not demand.
Let Z = rainfall in dairy-producing regions. Is Z a valid instrument?
Plausibly: whether it rains in dairy-producing regions shouldn’t affect demand.
Plausibly: insufficient rainfall means less grazing means less butter.
Z on arises because price and quantity are determined by the interaction of demand and supply.i = raini = rainfall in dairy-producing regions.
isolates changes in log price that arise from supply (part of supply, at least).
The regression counterpart of using shifts in the supply curve to trace out the demand curve.
Here is a (hypothetical) instrument: on arises because price and quantity are determined by the interaction of demand and supply.
Zi = Quakei = 1 if hit by quake, = 0 otherwise.
Substitute in on arises because price and quantity are determined by the interaction of demand and supply.
Thus on arises because price and quantity are determined by the interaction of demand and supply.
Subtract from each side and we get,
Multiplying through by and making the approximation that yields:
where because the instrument is relevant by assumption.
Put these together: approximation that yields:
Who approximation that yields: wrote Appendix B of PhilipWright (1928)?
…this appendix is thought to have been written with or by his son, SewallWright, an important statistician. (SW, p.425)
Who were these guys and what’s their story?
Why or why not?
Panel data: approximation that yields:
Proposed instrumental variable:
For now, use data for 1995 only. approximation that yields:
First stage OLS regression:
= 4.63+.031SalesTaxi , n=48
Second stage OLS regression with correct,
heteroskedasticity-robust standard errors.
Z1i = general sales tax component only
Z2i = cigarette-specific tax component only
The coefficients are said to be: approximation that yields:
There are just enough instruments to estimate
There are more than enough instruments to estimate 1 . If so, you can test whether the instruments are valid (a test of the “overidentifying restrictions”) - we’ll return to this later.
There are too few enough instruments to If so, you need to get more instruments!
Z1i = general sales taxi
Z2i = cigarette specific taxi
TSLS estimates, Z = sales tax (m = 1) approximation that yields:
TSLS estimates, Z = sales tax, cig-only tax (m = 2)
Recall the two requirements for valid instruments:
At least one instrument must enter the population counterpart of the first stage regression.
All the instruments must be uncorrelated with the error term:
What happens if one of these requirements isn’t satisfied?
How can we check? And what do we do?
We will focus on a single included endogenous regressor.
First stage regression:
are either zero or nearly zero.
Consider the simplest case:
With weak instruments, the denominator is nearly zero.
Why does normal approximation fail? approximation that yields:
Let denomenator is approximately
, then in large samples,
If the instrument is irrelevant, , then is the sample average of the random variable ri , i = 1 , … ,n, which are i.i.d, have variance , and have a mean of zero.
is approximately distributed N(0, 1).
is the distribution of aS, where and S is the ratio of two random variables, each of which has a standard normal distribution. And because Xi and ui are correlated, these two normal random variables are correlated.
Regress X on Z1 , … ,Zm, W1 , … ,Wr .
Rule-of-thumb: If the first stage F-statistic is less than 10, then the set of instruments is weak.
where E(F) is the expectation of the first-stage F-statistic.
Consider the simplest case:
Suppose # instruments = m > # X’s = k (overidentified). estimator , and let denote the asympototic bias of the OLS estimator.
The J -test of overidentifying restrictions
J = mF, where F = the F-statistic testing the coefficients on Z1i , … ,Zmi in a regression of the TSLS residuals against Z1i , … ,Zmi , W1i , … , Wri .
Distribution of the J-statistic
The two requirements for valid instruments:
Why are we interested in knowing the elasticity of demand for cigarettes?
(a) The “n-1 binary indicators” method
Instruments Z1it = general sales taxit
Z2it = cigarette－specific taxit
10-year change in log price = .
Elasticity = -.94, SE = .21.
Overall, this is a credible estimate of the long-term elasticity of demand although some problems might remain.
Does cardiac catheterization (心導管) improve longevity of heart attack patients?
Yi= survival time (in days) of heart attack patient
Xi= 1 if patient receives cardiac catheterization,
= 0 otherwise.
Figlio, David N. (2005), “Boys Named Sue: Disruptive Children and Their Peers,” NBERWorking Paper 11277.
X = measure of how disruptive your classmate’s are
Z = fraction of male classmates with female names
Feyrer, James, and Bruce Sacerdote (2007), “Colonialism and Modern Income— Islands as Natural Experiments,” Review of Economics and Statistics, forthcoming.
Y = log GDP per capita or infant mortality
X = number of years under colonial rule
Z = 12-month average of east-west wind speed,
12-month std. deviation of east-west wind speed
W = geographic dummies, area, latitude