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Unformatted text preview: TA session 5 Econ. 103, winter 2010 Wed., Feb. 3, 2010, 10:00 a.m. and 1:00 p.m. in PP2400E. 1 Verbose implementation of HW2 Q5 1.1 Summary stats for Californians • count if ca==1 – gives the count of those who say they are Californians: 99 • sum colgpa if ca==1 – gives the summary stats requested. You should see: Variable  Obs Mean Std. Dev. Min Max+ colgpa  95 3.528547 .2569961 2.8 3.94 Why are there 95 observations of colgpa for Californians but a total of 99 who say they are from California? It is because some people did not report their GPA. Try typing count if ca==1 & colgpa==. The ampersand & is used to express the logical “and” when you give Stata logical con ditions for the execution of some command. For example, try sum age if colgpa>=3.5 & ca==1 . 1.2 Summary stats for outofstaters • count if ca==0 – gives the count of those who say they aren’t Californians: 27 • sum colgpa if ca==0 – gives the summary stats requested. You should see: Variable  Obs Mean Std. Dev. Min Max+ colgpa  27 3.612481 .2987732 3 4 1.3 Confidence interval for the difference of the two populations’ expected college GPA Recall a confidence interval is constructed from estimates of the mean and standard deviation. A critical value c for the confidence interval is chosen as a function of the level of significance α . The confidence interval is wider for big c and smaller for small c . Its formula is bracketleftbigg ˆ μ c · ˆ σ √ n , ˆ μ + c · ˆ σ √ n bracketrightbigg I gave you a table in notes 43 to help you decide from which distribution we select the critical value. In practice, since the sample size is almost always n ≥ 30, we draw it from the normal. And you do not have to think about this anyway since Stata does it for you. 1 • ttest colgpa, by(ca) level(90) – this conducts a ttest and, along the way, it also gives you pvalues and forms the confidence interval. All in all, a useful command: Twosample t test with equal variances Group  Obs Mean Std. Err. Std. Dev. [90% Conf. Interval]+...
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 Winter '07
 SandraBlack
 Statistics, Econometrics, Standard Deviation, Obs Mean Std

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