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†††††† log:† C:\AAA Miker Files\newer web pages\soc_meth_proj3\class6_2009.log

† log type:† text

†opened on:† 12 Feb 2009, 11:15:35

 

. set mem 200m

 

Current memory allocation

 

††††††††††††††††††† current†††††††††††††††††††††††††††††††† memory usage

††† settable††††††††† value†††† description†††††††††††††††† (1M = 1024k)

††† --------------------------------------------------------------------

††† set maxvar†††††† ††5000†††† max. variables allowed†††††††††† 1.909M

††† set memory††††††††† 200M††† max. data space††††††††††††††† 200.000M

††† set matsize†††††††† 400†††† max. RHS vars in models††††††††† 1.254M

††††††††††††††††††††††††††††††††††††††††††††††††††††††††† ††-----------

†††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††† 203.163M

 

. use "C:\AAA Miker Files\newer web pages\soc_meth_proj3\cps_mar_2000_new.dta", clear

 

. table occ1990 if occ1990==178|occ1990==95, contents(mean incwage sd incwage freq)

 

---------------------------------------------------------------

Occupation, 1990† |

basis†††††††††††† | mean(incwage)††† sd(incwage)††††††††† Freq.

------------------+--------------------------------------------

Registered nurses |†† 37536.85197†††††† 21839.96††††††††††† 966

††††††††† Lawyers |†† 74044.32653†††††† 69032.96††††††††††† 441

---------------------------------------------------------------

 

. ttest incwage if occ1990==178|occ1990==95, by(occ1990)

 

Two-sample t test with equal variances

------------------------------------------------------------------------------

†† Group |†††† Obs††††††† Mean††† Std. Err.†† Std. Dev.†† [95% Conf. Interval]

---------+--------------------------------------------------------------------

Register |†††† 966††† 37536.85††† 702.6892††† 21839.96††† 36157.88††† 38915.83

†Lawyers |†††† 441††† 74044.33††† 3287.284††† 69032.96†††† 67583.6††† 80505.06

---------+--------------------------------------------------------------------

combined |†† †1407††† 48979.49††† 1223.363††† 45888.34††† 46579.68††† 51379.31

---------+--------------------------------------------------------------------

††† diff |†††††††††† -36507.47††† 2451.758†††††††††††††† -41316.97†† -31697.97

------------------------------------------------------------------------------

††† diff = mean(Register) - mean(Lawyers)†††††††††††††††††††††††† t = -14.8903

Ho: diff = 0†††††††††††††††††††††††††††††††††††† degrees of freedom =†††† 1405

 

††† Ha: diff < 0†††††††††††††††† Ha: diff != 0†††††††††††††††† Ha: diff > 0

†Pr(T < t) = 0.0000†††††††† Pr(|T| > |t|) = 0.0000††††††††† Pr(T > t) = 1.0000

 

. ttest incwage if occ1990==178|occ1990==95, by(occ1990) unequal

 

Two-sample t test with unequal variances

------------------------------------------------------------------------------

†† Group |†††† Obs††††††† Mean††† Std. Err.†† Std. Dev.†† [95% Conf. Interval]

---------+--------------------------------------------------------------------

Register |†††† 966††† 37536.85††† 702.6892††† 21839.96††† 36157.88††† 38915.83

†Lawyers |†††† 441††† 74044.33††† 3287.284††† 69032.96†††† 67583.6††† 80505.06

---------+--------------------------------------------------------------------

combined |††† 1407††† 48979.49††† 1223.363††† 45888.34††† 46579.68††† 51379.31

---------+--------------------------------------------------------------------

††† diff |†††††††††† -36507.47††† 3361.548†††††††††††††† -43112.62†† -29902.33

------------------------------------------------------------------------------

††† diff = mean(Register) - mean(Lawyers)†††††††††††††††††††††††† t = -10.8603

Ho: diff = 0†††††††††††††††††††† Satterthwaite's degrees of freedom =† 480.671

 

††† Ha: diff < 0†††††††††††††††† Ha: diff != 0†††††††††††††††† Ha: diff > 0

†Pr(T < t) = 0.0000†††††††† Pr(|T| > |t|) = 0.0000††††††††† Pr(T > t) = 1.0000

 

. *The unequal t-statistic is the formula we used before. The equal variance t-statistic is the default assumption, and it is also the t-statistic generated by standard regressions.

 

. gen byte lawyers=0

 

. replace lawyers=1 if occ1990==178

(441 real changes made)

 

. regress incwage lawyers if occ1990==178|occ1990==95

 

††††† Source |†††††† SS†††††† df†††††† MS††††††††††††† Number of obs =††† 1407

-------------+------------------------------†††††††††† F(† 1,† 1405) =† 221.72

†††††† Model |† 4.0354e+11†††† 1† 4.0354e+11†††††††††† Prob > F††††† =† 0.0000

††† Residual |† 2.5571e+12† 1405† 1.8200e+09†††††††††† R-squared†††† =† 0.1363

-------------+------------------------------†††††††††† Adj R-squared =† 0.1357

†††††† Total |† 2.9607e+12† 1406† 2.1057e+09†††††††††† Root MSE††††† =†† 42662

 

------------------------------------------------------------------------------

†††† incwage |††††† Coef.†† Std. Err.††††† t††† P>|t|††† †[95% Conf. Interval]

-------------+----------------------------------------------------------------

†††† lawyers |†† 36507.47†† 2451.758††† 14.89†† 0.000†††† 31697.97††† 41316.97

†††††† _cons |†† 37536.85†† 1372.618††† 27.35†† 0.000†††† 34844.25††† 40229.45

------------------------------------------------------------------------------

 

. *note that standard OLS regression gives us the equal variance t-statistic, 14.89

 

. display ttail(25,2.49)

.00989098

 

. *Just as with Freedman's table, this gives us 1% tail. But we generally want two-tailed tests, so we double it to 2%

 

. display 2*ttail(25,2.49)

.01978195

 

. display 2*ttail(1400,2.49)

.01288957

 

. *Instead of 2% we would have a 2-tail probability of 1.3% if we had N of 1400 instead of N of 25

 

. display 2*ttail(1400,14.89)

1.167e-46

 

. display normal(1.96)

.9750021

 

. *the normal function gives you the cumulative normal density up to the value, leaving one tail. The one-tail probability for Z-score of 1.96 is 2.5%, the two-tail probability is 5%

 

. display 2*(1-normal(1.96))

.04999579

 

. save "C:\AAA Miker Files\newer web pages\soc_meth_proj3\cps_mar_2000_new.dta", replace

file C:\AAA Miker Files\newer web pages\soc_meth_proj3\cps_mar_2000_new.dta saved

 

. *how does the two tail probability of the normal differ from the two tail probability of the T-statistic? It depends on N. Check Freedmanís table...

 

. exit, clear