
Two-sample t test with equal variances
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   Group |     Obs        Mean    Std. Err.   Std. Dev.   [95% Conf. Interval]
---------+--------------------------------------------------------------------
 control |  17,281    21579.59    168.8552    22197.24    21248.62    21910.56
experime |  17,281    22068.65    177.2784    23304.52    21721.17    22416.14
---------+--------------------------------------------------------------------
combined |  34,562    21824.12    122.4182     22758.6    21584.18    22064.06
---------+--------------------------------------------------------------------
    diff |           -489.0637    244.8259               -968.9304   -9.197017
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    diff = mean(control) - mean(experime)                         t =  -1.9976
Ho: diff = 0                                     degrees of freedom =    34560

    Ha: diff < 0                 Ha: diff != 0                 Ha: diff > 0
 Pr(T < t) = 0.0229         Pr(|T| > |t|) = 0.0458          Pr(T > t) = 0.9771
