
Two-sample t test with equal variances
------------------------------------------------------------------------------
   Group |     Obs        Mean    Std. Err.   Std. Dev.   [95% Conf. Interval]
---------+--------------------------------------------------------------------
 control |  17,281       21715    171.3572    22526.14    21379.13    22050.88
experime |  17,281    21933.24     174.877    22988.84    21590.46    22276.02
---------+--------------------------------------------------------------------
combined |  34,562    21824.12    122.4182     22758.6    21584.18    22064.06
---------+--------------------------------------------------------------------
    diff |           -218.2355    244.8372               -698.1244    261.6534
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    diff = mean(control) - mean(experime)                         t =  -0.8913
Ho: diff = 0                                     degrees of freedom =    34560

    Ha: diff < 0                 Ha: diff != 0                 Ha: diff > 0
 Pr(T < t) = 0.1864         Pr(|T| > |t|) = 0.3727          Pr(T > t) = 0.8136
