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From |
simone ferro <simone.ferro88@gmail.com> |

To |
"statalist@hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu> |

Subject |
st: problem with the interpretation of pstest after psmatch2, t-tests and percentage of bias provide conflicting results, which one should I follow? |

Date |
Sat, 12 Jan 2013 13:37:15 +0100 |

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/

**Follow-Ups**:**Re: st: problem with the interpretation of pstest after psmatch2, t-tests and percentage of bias provide conflicting results, which one should I follow?***From:*Adam Olszewski <adam.olszewski@gmail.com>

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