# Can the Mann-Whitney Test be used with small n or loose independence?

I have data on market share. I would like to compare a firm's mean market share before and after an event.

The data is quarterly and I have only been provided with 16 observations, 8 before the event and 8 after the event.

The data looks to be non-normal (most likely due to small number of observations).

The variances are not equal.

So in this case, am I able to use the Mann-Whitney test?

I worry that:

(1)There are not enough observations (2)The data is from a single firm so may not be truly independent

Are these okay to ignore or should I be looking for a different test?

Thank you for having a look.

The sample size is small, but it is not so small as to rule out use of the Mann-Whitney test.  However, if you are concerned, it would be reasonable to do a boot-strap analysis (shuffling the data many times and calculating the statistic).  The resulting $P$-value from this analysis may be more informative.