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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
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