Quote:
Originally Posted by WoodlandBrew
Sometime I just need a ball park number and might only count 100300 cells. These have an average box to box standard deviation of 6%.

1/sqrt(100) = 10%; 1/sqrt(300) = 5.77%. Ain't science grand.
Quote:
Originally Posted by WoodlandBrew
If I take my time and do everything right and count 500 cells or more the standard deviation is 3% on average.

To cut your 300 count population sd in half you'd have to count 4*300 = 1200 cells. It's entirely possible that you get, by luck of the draw, a sample standard deviation less than the population sd but that won't happen very often. I question 3% in the long term. Yes, it can happen but it's unlikely.
Quote:
Originally Posted by WoodlandBrew
It is quite possible to have a very low standard deviation but have the numbers be way off. Recently I had some alcohol left in a tube from cleaning and didn't realize that it had not completely evaporated. My viability was much lower than expected.

That's called a 'bias error'. If I snuck into your lab and replaced your 0.0001 mL heomocytometer with one that had a volume of 0.00009 mL all your counts would be 10% low but your variances would be little changed.