Main > Definitions>What is Sample Size?

## 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.

- If you