先用了t.test做检验， 因为觉得小数位太长，用了options(digits=3)。然后发现，t.test前后的自由度大不一样了，这是为什么呀？

一开始的结果：
Welch Two Sample t-test

data:  Prob by So
t = -3.9, df = 24.9, p-value = 0.00065
alternative hypothesis: true difference in means is not equal to 0
95 percent confidence interval:
-0.038526 -0.011874
sample estimates:
mean in group 0 mean in group 1
       0.038513        0.063713

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options（digits3）后的结果：
  Welch Two Sample t-test

data:  Prob by So
t = -4, df = 20, p-value = 7e-04
alternative hypothesis: true difference in means is not equal to 0
95 percent confidence interval:
-0.039 -0.012
sample estimates:
mean in group 0 mean in group 1
          0.039           0.064

还有，为什么统计量都不是保留小数3位，t值取整了？