# 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...

-Stan_-

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...

-HomeBrew-

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...

-Stan_-

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.

-HomeBrew-