>count does handle masked data, so in that sense maybe a tolerance does make sense since it would signify a simple level of confidence in the returned value.

I'm really not sure in what scenario that would be useful from a scientific perspective, but I understand that technically it does accept masks so it should work. 

>However, you have a point in that MEAN is the real targeted reason for this enhancement so there should really be an integration test here specifically for iris.analysis.MEAN.

Indeed. I would be more confident if the test used MEAN or similar.