
Two-sample t test with equal variances
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   Group |     Obs        Mean    Std. Err.   Std. Dev.   [95% Conf. Interval]
---------+--------------------------------------------------------------------
 control |  17,281    21738.42    172.5486    22682.76     21400.2    22076.63
experime |  17,281    21909.83    173.7031    22834.53    21569.35     22250.3
---------+--------------------------------------------------------------------
combined |  34,562    21824.12    122.4182     22758.6    21584.18    22064.06
---------+--------------------------------------------------------------------
    diff |           -171.4094    244.8383               -651.3004    308.4816
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    diff = mean(control) - mean(experime)                         t =  -0.7001
Ho: diff = 0                                     degrees of freedom =    34560

    Ha: diff < 0                 Ha: diff != 0                 Ha: diff > 0
 Pr(T < t) = 0.2419         Pr(|T| > |t|) = 0.4839          Pr(T > t) = 0.7 581
