
Two-sample t test with equal variances
------------------------------------------------------------------------------
   Group |     Obs        Mean    Std. Err.   Std. Dev.   [95% Conf. Interval]
---------+--------------------------------------------------------------------
 control |  17,281    21800.37    175.0853    23016.22    21457.18    22143.55
experime |  17,281    21847.88    171.1482    22498.67    21512.41    22183.34
---------+--------------------------------------------------------------------
combined |  34,562    21824.12    122.4182     22758.6    21584.18    22064.06
---------+--------------------------------------------------------------------
    diff |           -47.50901    244.8399               -527.4031    432.3851
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    diff = mean(control) - mean(experime)                         t =  -0.1940
Ho: diff = 0                                     degrees of freedom =    34560

    Ha: diff < 0                 Ha: diff != 0                 Ha: diff > 0
 Pr(T < t) = 0.4231         Pr(|T| > |t|) = 0.8461          Pr(T > t) = 0.5769
