# How to determine a sample # - (Jul/30/2010 )

Hi,

I am doing a project in determining if there is a statistically significant difference between conducting a particular type of cleaning process and not doing that cleaning process on food production equipments. So I am collecting swabs on the equipment before and after the application of this process and analyzing the swabs for Aerobic Plate Count. My question is how many samples would be a good sample number to make some statistical sense. There are about 3 locations that I am sampling before and after the process. Do you guys have any suggestions...

Thanks for your help.

It depends on the confidence level you want to report your results at, and what confidence interval you're comfortable with. See here, for example...

Okay lets say a confidence level of 95%. I don't understand what will be the population here either.

HomeBrew on Fri Jul 30 20:14:06 2010 said:

It depends on the confidence level you want to report your results at, and what confidence interval you're comfortable with. See here, for example...

According to a similar calculator here, population is:

How many people are there in the group your sample represents? This may be the number of people in a city you are studying,the number of people who buy new cars, etc. Often you may not know the exact population size. This is not a problem. The mathematics of probability proves the size of the population is irrelevant, if the size of the sample exceeds a few percent of the total population you are examining. This means that a sample of 500 people is equally useful in examining the opinions of a state of 15,000,000 as it would a city of 100,000. For this reason the population size is ignored when population is large or unknown. Population size is only likely to be a factor when you work with a relatively small and known group of people.