# What are the assumptions (and H0) for Wilcoxon signed-rank test?

I am working on the assumption page of the Wilcoxon signed-rank test (the wilcox "paired test") in Wikipedia.

I was able to locate a reference for the assumption that I wrote there, which are:

Let $Z_i=X_i – Y_i$ for $i = 1, \ldots , n$.

1. The differences $Z_i$ are assumed to be independent.
2. Each $Z_i$ comes from the same continuous population, and is symmetric about a common median θ .
3. The values which $X_i$ and $Y_i$ represent are ordered (at least the ordinal level of measurement), so the comparisons "greater than", "less than", and "equal to" are useful.

However, I get this nagging feeling that I am missing something here. Can anyone correct/expand on this?

Thanks.

• To take differences you need more than ordinal scale. Jun 28, 2013 at 12:51