Using Mann-Whitney test when all values in one data set are zero (SPSS vs Graphpad)
I would be very grateful for advice on the following problem:
I am examining the frequency in which political issues are reported in the national media in the six days before and six days after a particular date. An example of the data set for one issue in the six days beforehand is 8,7,9,5,6,7 i.e. on day one the issue is reported in 8 newspapers.
I am comparing the before/after data sets using the Mann-Whitney U test to see if there is a significant difference in reporting, as I cannot assume normal distribution.
The problem is that there are some issues not reported at all before the date, but reported heavily in the six days after (e.g. 0,0,0,0,0,0 vs 8,7,8,4,6,8). Graphpad will not let me use the Mann-Whitney test when all of the values in one data set are zero, and insists on the Wilcoxon Rank Sum test instead. However, SPSS has no problems with the Mann-Whitney test.
Could I please have some advice as to which test is the most appropriate