If you take a population sample, you must use a formula to figure out what sample size you need to take. Sometimes you know something about a population, which can help you determine a sample size. For example, it’s well known that IQ scores follow a normal distribution pattern. But what about if you know nothing about your population at all? That’s when you can use Slovin’s formula to figure out what sample size you need to take, which is written as n = N / (1 + Ne2) where n = Number of samples, N = Total population and e = Error tolerance
Sample question: Use Slovin’s formula to find out what sample of a population of 1,000 people you need to take for a survey on their soda preferences.
Step 1: Figure out what you want your confidence level to be. For example, you might want a confidence level of 95 percent (which will give you a margin error of 0.05), or you might need better accuracy at the 98 percent confidence level (which produces a margin of error of 0.02).
n = N / (1 + N e2) =
1,000 / (1 + 1000 * 0.05 2) = 285.714286
Step 3: Round your answer to a whole number (because you can’t sample a fraction of a person or thing!)
285.714286 = 286