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I have the number of dispensed items per year and the cost per item - the graphs look as if when the number of dispensed items increase the cost comes down per item (red = cost, blue = number dispensed). For example,
I want to do some a statistical test to see whether or not the number of items dispensed in a year increasing forces the cost down / if they are correlated, but I'm not sure which test to use.
If the assumptions of randomness, fixed distribution, fixed location, and fixed variation hold, you can perform an independent samples t-test. If the assumptions are violated, try a Mann-Whitney test.
If you are just interested in correlation, try a Pearson test. If the assumptions are violated, try a Spearman test.
If you would like to perform a statistical test with time series in mind, consider a Granger causality test.
