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SAMPLE SELECTION BIAS In the tobit model, the values of the regressors and the disturbance term determine whether or not an observation falls into the participating category (Y > 0) or the non-participating category (Y = 0). 1

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SAMPLE SELECTION BIAS However, the decision to participate may depend on factors other than those in the regression model, in which case a more general model specification with a two-stage process may be required. 2

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SAMPLE SELECTION BIAS The first stage, the selection process (decision to participate), depends on the net benefit of participating, B*, a latent (unobservable) variable that depends on a set of variables Qj and a random term e: 3

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**Yi unobserved if SAMPLE SELECTION BIAS for**

The second stage, the regression model, is parallel to that for the tobit model. The X variables may include some of the Q variables. A sufficient condition for identification is that at least one Q variable is not also an X variable. 4

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**Yi unobserved if SAMPLE SELECTION BIAS for**

The expected value of u for an observation in the sample is its expected value conditional on B* > 0 (if this condition is not satisfied, the observation will not be in the sample). 5

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**Yi unobserved if SAMPLE SELECTION BIAS for**

B* will be greater than 0 if e satisfies the inequality shown. 6

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**Yi unobserved if SAMPLE SELECTION BIAS for**

It can be shown that E(ui) is equal to se u, the population covariance of e and u, divided by se, the standard deviation of e, multiplied by li, defined as shown. 7

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**Yi unobserved if SAMPLE SELECTION BIAS for**

li is usually described as the inverse of Mills' ratio. f is the density function of the standardized normal distribution and F is the cumulative standardized normal distribution. 8

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**Yi unobserved if SAMPLE SELECTION BIAS for**

The nonstochastic component of Y in observation i is, as usual, its expected value. In this model the expected value must take into account the condition that the observation appears in the sample. 9

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**Yi unobserved if SAMPLE SELECTION BIAS for**

Substituting for Yi, we obtain the expression shown. 10

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**Yi unobserved if SAMPLE SELECTION BIAS for**

The first two components of Yi are not affected by taking expectations. The expected value of ui is as shown above. 11

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**Yi unobserved if SAMPLE SELECTION BIAS for**

If the random component of the selection process is distributed independently of the random component of the function for Y, the population covariance of e and u will be 0 and the term involving the inverse of Mills' ratio drops out. 12

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**Yi unobserved if SAMPLE SELECTION BIAS for**

In that case we could use least squares to fit the model as usual. 13

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**Yi unobserved if SAMPLE SELECTION BIAS for**

However, in some situations it is reasonable to hypothesize that the random components are not distributed independently because some of the unobserved characteristics affecting Y* also affect the selection process. 14

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**Yi unobserved if SAMPLE SELECTION BIAS for**

Observations in the sample will then be systematically different from those not in the sample and the model is said to be subject to sample selection bias. 15

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**Yi unobserved if SAMPLE SELECTION BIAS for**

Effectively, this is a special type of omitted variable bias. A regression of Y on the X variables will yield inconsistent estimates caused by the omission of the li term. Note that li has different values in different observations and is therefore a special type of variable. 16

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**Yi unobserved if SAMPLE SELECTION BIAS for**

However, since its components depend only on the selection process, li can be estimated from the results of probit analysis of the selection process. This is the first step of the Heckman two-step procedure. 17

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**Yi unobserved if SAMPLE SELECTION BIAS for**

The second step is to regress Y on the X variables and the estimated l. 18

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SAMPLE SELECTION BIAS . heckman LGEARN S ASVABC ETHBLACK ETHHISP if MALE==0, select(S AGE CHILDL06 CHILDL16 MARRIED ETHBLACK ETHHISP) We will illustrate the heckman procedure by fitting an earnings function for females using the LFP data set on the website. The includes 2,661 females, of whom 2,021 had earnings, in 1994. 19

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**SAMPLE SELECTION BIAS In Stata the regression command is 'heckman'.**

. heckman LGEARN S ASVABC ETHBLACK ETHHISP if MALE==0, select(S AGE CHILDL06 CHILDL16 MARRIED ETHBLACK ETHHISP) In Stata the regression command is 'heckman'. 20

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SAMPLE SELECTION BIAS . heckman LGEARN S ASVABC ETHBLACK ETHHISP if MALE==0, select(S AGE CHILDL06 CHILDL16 MARRIED ETHBLACK ETHHISP) It is followed by the dependent variable and the explanatory variables in the regression. Note that we are restricting the sample to females. 21

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SAMPLE SELECTION BIAS . heckman LGEARN S ASVABC ETHBLACK ETHHISP if MALE==0, select(S AGE CHILDL06 CHILDL16 MARRIED ETHBLACK ETHHISP) After a comma, the selection process is specified using 'select' followed by the selection variables in parentheses. 22

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SAMPLE SELECTION BIAS . heckman LGEARN S ASVABC ETHBLACK ETHHISP if MALE==0, select(S AGE CHILDL06 CHILDL16 MARRIED ETHBLACK ETHHISP) CHILDL06 is a dummy equal to 1 if there is a child aged less than 6. CHILDL16 is 1 if there is a child aged less than 15 and CHILDL06 is 0. MARRIED is equal to 1 if the respondent is married with spouse present. Otherwise the variables are as defined in the EAEF data sets. 23

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SAMPLE SELECTION BIAS . heckman LGEARN S ASVABC ETHBLACK ETHHISP if MALE==0, select(S AGE CHILDL06 CHILDL16 MARRIED ETHBLACK ETHHISP) Iteration 0: log likelihood = (not concave) Iteration 1: log likelihood = (not concave) Iteration 2: log likelihood = (not concave) Iteration 3: log likelihood = (not concave) Iteration 4: log likelihood = Iteration 5: log likelihood = Iteration 6: log likelihood = Iteration 7: log likelihood = Iteration 8: log likelihood = Since the model involves probit analysis, it is fitted using maximum likelihood estimation. 24

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SAMPLE SELECTION BIAS Heckman selection model Number of obs = (regression model with sample selection) Censored obs = Uncensored obs = Wald chi2(4) = Log likelihood = Prob > chi = | Coef. Std. Err z P>|z| [95% Conf. Interval] LGEARN | S | ASVABC | ETHBLACK | ETHHISP | _cons | select | S | AGE | CHILDL06 | CHILDL16 | MARRIED | ETHBLACK | ETHHISP | _cons | The numbers of participating and non-participating respondents are given at the top of the output. 25

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**SAMPLE SELECTION BIAS Next comes the regression output.**

Heckman selection model Number of obs = (regression model with sample selection) Censored obs = Uncensored obs = Wald chi2(4) = Log likelihood = Prob > chi = | Coef. Std. Err z P>|z| [95% Conf. Interval] LGEARN | S | ASVABC | ETHBLACK | ETHHISP | _cons | select | S | AGE | CHILDL06 | CHILDL16 | MARRIED | ETHBLACK | ETHHISP | _cons | Next comes the regression output. 26

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SAMPLE SELECTION BIAS Heckman selection model Number of obs = (regression model with sample selection) Censored obs = Uncensored obs = Wald chi2(4) = Log likelihood = Prob > chi = | Coef. Std. Err z P>|z| [95% Conf. Interval] LGEARN | S | ASVABC | ETHBLACK | ETHHISP | _cons | select | S | AGE | CHILDL06 | CHILDL16 | MARRIED | ETHBLACK | ETHHISP | _cons | The results of the probit analysis of the selection process follow. 27

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SAMPLE SELECTION BIAS /athrho | /lnsigma | rho | sigma | lambda | LR test of indep. eqns. (rho = 0): chi2(1) = Prob > chi2 = The final part of the output gives the selection bias statistics. rho gives an estimate of the correlation between e and u, here 0.77. 28

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SAMPLE SELECTION BIAS /athrho | /lnsigma | rho | sigma | lambda | LR test of indep. eqns. (rho = 0): chi2(1) = Prob > chi2 = For technical reasons, r is estimated indirectly via atanh r. However, a test of the null hypothesis H0: atanh r = 0 is equivalent to a test of the null hypothesis of H0: r = 0. 29

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SAMPLE SELECTION BIAS /athrho | /lnsigma | rho | sigma | lambda | LR test of indep. eqns. (rho = 0): chi2(1) = Prob > chi2 = The asymptotic t statistic is and so the null hypothesis is rejected. 30

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**SAMPLE SELECTION BIAS LU = –2668.81**

Heckman selection model Number of obs = (regression model with sample selection) Censored obs = Uncensored obs = Wald chi2(4) = Log likelihood = Prob > chi = | Coef. Std. Err z P>|z| [95% Conf. Interval] LGEARN | S | ASVABC | ETHBLACK | ETHHISP | _cons | select | S | AGE | CHILDL06 | CHILDL16 | MARRIED | ETHBLACK | ETHHISP | _cons | LU = – An alternative test involves a comparison of the log likelihood for this model with that for a restricted version where r is assumed to be 0. 31

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**LR = log-likelihood imposing restriction r = 0**

SAMPLE SELECTION BIAS /athrho | /lnsigma | rho | sigma | lambda | LR test of indep. eqns. (rho = 0): chi2(1) = Prob > chi2 = LU = – LR = log-likelihood imposing restriction r = 0 Test statistic = 2(LR – LU) ~ c2(1) under H0: r = 0 The test statistic 2 (log LU – log LR), where log LU and log LR are the log-likelihoods for the unrestricted and restricted versions, is distributed as a chi-squared statistic with 1 degree of freedom under the null hypothesis that the restriction r = 0 is valid. 32

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**LR = log-likelihood imposing restriction r = 0**

SAMPLE SELECTION BIAS /athrho | /lnsigma | rho | sigma | lambda | LR test of indep. eqns. (rho = 0): chi2(1) = Prob > chi2 = LU = – LR = log-likelihood imposing restriction r = 0 Test statistic = 2(LR – LU) ~ c2(1) under H0: r = 0 c2(1)crit, 0.1% = 10.83 In this example the test statistic is The critical value of chi-squared with one degree of freedom at the 0.1 percent level is 10.83, so the null hypothesis is rejected. 33

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SAMPLE SELECTION BIAS . heckman LGEARN S ASVABC ETHBLACK ETHHISP if MALE==0, select(S AGE CHILDL06 CHILDL16 MARRIED ETHBLACK ETHHISP) | Coef. Std. Err z P>|z| [95% Conf. Interval] LGEARN | S | ASVABC | ETHBLACK | ETHHISP | _cons | . reg LGEARN S ASVABC ETHBLACK ETHHISP if MALE==0 LGEARN | Coef. Std. Err t P>|t| [95% Conf. Interval] S | ASVABC | ETHBLACK | ETHHISP | _cons | It is instructive to compare the fitted earnings functions for the heckman and least squares models. The coefficients are fairly similar, despite the inconsistency of the least squares estimates. 34

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SAMPLE SELECTION BIAS . heckman LGEARN S ASVABC ETHBLACK ETHHISP if MALE==0, select(S AGE CHILDL06 CHILDL16 MARRIED ETHBLACK ETHHISP) | Coef. Std. Err z P>|z| [95% Conf. Interval] LGEARN | S | ASVABC | ETHBLACK | ETHHISP | _cons | . reg LGEARN S ASVABC ETHBLACK ETHHISP if MALE==0 LGEARN | Coef. Std. Err t P>|t| [95% Conf. Interval] S | ASVABC | ETHBLACK | ETHHISP | _cons | The coefficient of schooling is a little higher in the heckman regression. 35

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SAMPLE SELECTION BIAS Heckman selection model Number of obs = (regression model with sample selection) Censored obs = Uncensored obs = Wald chi2(4) = Log likelihood = Prob > chi = | Coef. Std. Err z P>|z| [95% Conf. Interval] LGEARN | S | ASVABC | ETHBLACK | ETHHISP | _cons | select | S | AGE | CHILDL06 | CHILDL16 | MARRIED | ETHBLACK | ETHHISP | _cons | The probit analysis showed that schooling has a highly significant positive effect on labor force participation, controlling for other characteristics such as number of children of school age. 36

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SAMPLE SELECTION BIAS . heckman LGEARN S ASVABC ETHBLACK ETHHISP if MALE==0, select(S AGE CHILDL06 CHILDL16 MARRIED ETHBLACK ETHHISP) | Coef. Std. Err z P>|z| [95% Conf. Interval] LGEARN | S | ASVABC | ETHBLACK | ETHHISP | _cons | . reg LGEARN S ASVABC ETHBLACK ETHHISP if MALE==0 LGEARN | Coef. Std. Err t P>|t| [95% Conf. Interval] S | ASVABC | ETHBLACK | ETHHISP | _cons | If females with higher levels of schooling are relatively keen to work, they will tend to be willing to accept lower wages, controlling for other factors including education, than those who are reluctant to work. 37

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SAMPLE SELECTION BIAS . heckman LGEARN S ASVABC ETHBLACK ETHHISP if MALE==0, select(S AGE CHILDL06 CHILDL16 MARRIED ETHBLACK ETHHISP) | Coef. Std. Err z P>|z| [95% Conf. Interval] LGEARN | S | ASVABC | ETHBLACK | ETHHISP | _cons | . reg LGEARN S ASVABC ETHBLACK ETHHISP if MALE==0 LGEARN | Coef. Std. Err t P>|t| [95% Conf. Interval] S | ASVABC | ETHBLACK | ETHHISP | _cons | Hence the wages of more-educated females will tend not to reflect the full value of education in the market place. The least squares regression does not take account of this, and hence the estimate of the return to schooling is lower. 38

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**Copyright Christopher Dougherty 2012.**

These slideshows may be downloaded by anyone, anywhere for personal use. Subject to respect for copyright and, where appropriate, attribution, they may be used as a resource for teaching an econometrics course. There is no need to refer to the author. The content of this slideshow comes from Section 10.5 of C. Dougherty, Introduction to Econometrics, fourth edition 2011, Oxford University Press. Additional (free) resources for both students and instructors may be downloaded from the OUP Online Resource Centre Individuals studying econometrics on their own who feel that they might benefit from participation in a formal course should consider the London School of Economics summer school course EC212 Introduction to Econometrics or the University of London International Programmes distance learning course 20 Elements of Econometrics

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