N1=240 N2=260
Also a reference to your answer would be great please!
N1=240 N2=260
Also a reference to your answer would be great please!
Yes. See: original paper by Mann and Whitney.
Specifically, note the last sentence in their abstract referring to two distinct sample sizes.
Let $x$ and $y$ be two random variables with continuous cumulative distribution functions $f$ and $g$. A statistic $U$ depending on the relative ranks of the $x$'s and $y$'s is proposed for testing the hypothesis $f=g$. Wilcoxon proposed an equivalent test in the Biometrics Bulletin, December, 1945, but gave only a few points of the distribution of his statistic. Under the hypothesis $f=g$ the probability of obtaining a given $U$ in a sample of $n$ $x$'s and $m$ $y$'s is the solution of a certain recurrence relation involving $n$ and $m$. [...]