Calculating Sample Sizes For Marketing Research Surveys

One of the more fundamental questions all researchers must answer before launching a survey is how many people do you want to survey. How big must your sample size be?

To determine how many people to survey, first you need to decide how much margin of error you can tolerate in the results. To put this another way: how accurate do you want your data to be?

The more people you survey, the more accurate your results will be. If you interview everyone in your target population, then you can say with complete accuracy what that population thinks.

But surveying everyone usually is prohibitively expensive, as well as unnecessary.

You can interview a sample, a portion of the total population, and project those results with some degree of accuracy onto the total population. But you have to expect some variation, or error, in the answers you get from the sample and the answers you might have gotten if you had actually surveyed everybody.

The margin of error tells you the range in which the "true" answer is likely to be found. The confidence interval tells you the probability that the "true" answer will fall within the range of the margin of error.

For instance, if the margin of error is plus or minus five percent and the confidence level is 95 percent, you can say this about your data: you can be certain that 95 percent of the time you will get answers that are within five percent (up or down) that you would have gotten had you surveyed the entire population.

After you know how accurate you want your data to be and you've determined the size of the population you want to get information on, then you must determine whether the results of your survey will be in proportions or percentages, or in means or averages. With that information, you can easily calculate the appropriate sample size.

Note: keep in mind that margin of error is inversely proportional to cost - the higher the margin of error, the less accurate the results, but the smaller the sample and therefore the cheaper the survey costs. Since budget always plays a role in market research project decisions, you have to go with what is reasonable given your circumstances.