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The question is to compare means of sales volume in different periods. So I got sales period 1 and sales period 2. Sample size is 40. The skewness, kurtosis, test of normality and histogram all show sales period 1 has a normal distribution but sales period 2 does not. Should I use paired samples t test or Wilcoxon? I did both test but the results are compeletely different.

Sherry
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    If the data are paired the individual distributions wouldn't need to be normal. Which Wilcoxon test did you use? What does the distribution of pair differences look like (say in a normal Q-Q plot)? What do the individual values consist of? Would you expect the changes under the alternative to consist of noise around constant shifts or more like noisy percentage changes around a typical percentage change? – Glen_b Oct 25 '14 at 06:30
  • This link might help: http://stats.stackexchange.com/questions/2492/is-normality-testing-essentially-useless – rnso Oct 25 '14 at 09:09

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