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Good luck, Adam Olszewski On Sat, Jan 12, 2013 at 7:37 AM, simone ferro <simone.ferro88@gmail.com> wrote: > dear Statalist, > > I would please need some clarifications about the interpretation of the command pstest after running psmatch2: > I report a random output just as an example. > > pstest ebitda_marg asset employees av_age donne_perc laureati_perc bluecollar_perc, both > > ------------------------------------------------------------------------------ > Unmatched | Mean %reduct | t-test > Variable Matched | Treated Control %bias |bias| | t p>|t| > --------------------------+----------------------------------+---------------- > ebitda_marg Unmatched | 11.404 8.2304 36.8 | 3.25 0.001 > Matched | 10.746 10.555 2.2 94.0 | -2.33 0.020 > | | > asset Unmatched | 395.52 34.389 28.6 | 2.48 0.014 > Matched | 115.36 89.157 2.1 92.7 | -2.10 0.037 > | | > employees Unmatched | 641.98 508.48 4.6 | 0.42 0.677 > Matched | 474.12 704.43 -8.0 -72.5 | 0.18 0.857 > | | > av_age Unmatched | 53.369 56.714 -45.0 | -4.06 0.000 > Matched | 53.39 53.051 4.6 89.9 | 3.84 0.000 > | | > donne_perc Unmatched | .34711 .37372 -11.8 | -1.06 0.291 > Matched | .34805 .33374 6.3 46.2 | 0.16 0.874 > | | > laureati_perc Unmatched | .26656 .18165 35.4 | 3.15 0.002 > Matched | .26815 .23665 13.2 62.9 | -1.52 0.129 > | | > bluecollar_~c Unmatched | .30019 .3349 -11.2 | -1.00 0.317 > Matched | .30425 .33592 -10.2 8.7 | -0.28 0.776 > | | > ------------------------------------------------------------------------------ > If I understood well, reported t-.tests' null hypothesis is that the two covariates are equal in treated and control group, > so I should look at t-tests to check if the groups are well balanced, > In some tutorial instead I've read that the right approach is to look at the bias percentage that should be under 10 to be considered ok, > > Which one of the two approaches is the right one?it's fundamental for me to understand because they provide totally different interpretations. > indeed if I look at t-tests, I find problems with ebitda_margin, asset and av_age, because they are significantly different in the two groups, > while if I look at the bias percentage, I find problem with laureati_perc and blecollar, because their bias% are bigger than 10. > > I also would appreciate your confirmation of the interpretation of the two indicators(%bias and t-tests), because with this interpretation I find the two values contradictory. > looking for example at the variable av_age, I find a very little bias, and the means after matching of treated and control group are almost identical(53.39 and 53.051), by the way the t-test reports a p-value of 0.000! > So it seems like I have misunderstood the meaning of the t.test because I don't think that 53.39 and 53.051 can be statistically different with a t-stat of 3.84, also given the nature of the variable(the average age of the managers of a firm) that should infact be quite variable. > The same happens with the variable ebitda, which reports a bias% of 2.2% and almost identical values(10.746 and 10.555), but t-stat is -2.33 and p-value 2%! > Can you please help me? > thanks in advance for the help, > Regards, > Simone Ferro > * > * For searches and help try: > * http://www.stata.com/help.cgi?search > * http://www.stata.com/support/faqs/resources/statalist-faq/ > * http://www.ats.ucla.edu/stat/stata/ * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/faqs/resources/statalist-faq/ * http://www.ats.ucla.edu/stat/stata/

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