Monday, September 25, 2017

How many people do we need to survey?

This is one of the more common questions asked by clients when discussing quantitative market research. The answer, unfortunately, is not always straightforward. Several factors determine the recommended sample size for a project. These include:

  • The desired confidence interval,
  • The desired confidence level,
  • The size of the population, and
  • The expected outcome

 

Sample Size Calculator - Simple

Use this calculator unless you have a small population and/or have an expected/measured outcome that is below 40% or above 60%.

Calculate 

       
These terms are defined below in the context of political polling:

Confidence Interval

The confidence interval is the plus-or-minus figure usually reported in newspaper or television opinion poll results. A politician's approval rating may be cited as 54% +/- 3%. In this example, the confidence interval is 3%. This means if you polled the entire population, you would expect that the actual approval rating would fall between 51% and 57%.  This is also referred to as the margin of error.

Confidence Level

The confidence level tells you how sure you can be that the true answer falls within the confidence interval. The confidence level is expressed as a percentage. The 95% confidence level means you can be 95% certain about your confidence interval, the 99% confidence level means you can be 99% certain. Higher confidence levels require larger sample sizes to achieve the same confidence interval.  Most researchers use the 95% confidence level.

 

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