I have two populations with n=18 and I'm trying to find out if it makes sense to compare them with a t-test. I ran a Shapiro-Wilk test in SigmaPlot 12.5 for both populations seperately and these are the results:
population1: W-Statistic = 0.900 P = 0.057 Passed
population2: W-Statistic = 0.912 P = 0.094 Passed
However, if I'm trying to run a t-test, it says:
Normality Test (Shapiro-Wilk) Failed (P = 0.003)
Here it seems that there is only one P-value for both populations, which is a bit confusing to me. Does anyone have an idea how the P-value might have been calculated here and why it can be that low, even if it is much higher for both populations tested seperately?
This is the underlying data...
pop1: pop2:
6.0696 6.4659
6.8833 6.2842
5.9243 5.9193
6.5391 7.526
7.2505 6.71
6.4299 7.3117
4.9903 13.5116
4.8506 9.1565
4.7737 7.7016
6.9384 8.5998
6.6842 9.2543
6.614 10.2234
6.3128 9.7079
6.3533 7.8677
6.2728 8.7079
7.4372 9.405
7.2657 8.5998
7.1165 8.3411