What is Sample Size: Overview
A sample size is a portion of the population selected for a survey. For example, you might take a survey of dog owner’s brand preferences. You won’t want to survey all the millions of dog owners in the country (either because it’s too expensive or time consuming), so you take a sample size. That may be several thousand owners. The sample size is a representation of all dog owner’s brand preferences.
What is a Sample Size: Sampling Error
When you only survey a small sample of the population, uncertainty creeps in to your statistics. If you can only survey a certain percentage of the true population, you can never be 100% sure that your statistics are a complete and accurate representation of the population. This uncertainty is called sampling error and is usually measured by a confidence interval. For example, you might state that your results are at a 90% confidence level. That means if you were to repeat your survey over and over, 90% of the time your would get the same results.
What is a Sample Size: Determining Sample Size
One of the most often asked questions surrounding surveys is “How do I determine sample size?” The answer is, it depends on what you know about the population.
Some general techniques for finding a sample size:
- Small populations: conduct a census.
- Use a sample size from a similar study.
- Use tables. For example, this article at the University of Florida shows tables for determining sample size for a 95% confidence level (scroll to the bottom of the article for the tables).
- Use a sample size calculator, like this one from the National Statistical Service in Australia.
- Use a formula. Which formula you use depends on what you know about your population.
- If you know the standard deviation of your population you can this technique: Find Sample Size Statistics: How to find a sample size given a confidence interval, width and known standard deviation.
- If you don’t know the standard deviation of your population, you can use this technique: Find Sample Size Statistics: How to find a sample size given a confidence interval, width and unknown population standard deviation.
- If you know nothing at all about your population, you might be able to use Slovin’s formula to find your sample size.