Sample Size for Community Needs & Resource Assessment Surveys

A small piece is enough. Like the cheesecake, sampling procedures allow one to survey a portion of the population and obtain data that can be considered nearly as reliable as if the whole population had been surveyed.

Determining the best sample size is not a simple process. The following factors influence sample size decisions:

• Size - The larger the population, the larger the sample size.

• Number of sub-groups - Many surveys target different sub-populations. For example, to separately analyze the results from surveys completed by the elderly, the sample size must meet the requirements for a specific level of accuracy for the general survey and for the subgroup of the elderly. To provide meaningful information on subgroups usually requires a larger sample size.

• Level of accuracy - Most people work for an accuracy level of .95 with a .05 confidence interval. This means that you are 95% certain the results are not off by more than 5%, plus or minus. The level of accuracy is directly proportional to the sample size. As the required standard of accuracy increases, so does the size of the required sample.

• Cost - Fixed costs do not vary with sample size. These include the cost to design the survey, analyze the results and write a final report. Fixed costs usually represent a major portion of the budget. Variable costs increase with increases in sample size. Variable costs include printing, collecting and entering data.

• Time - Time constraints may restrict the number of persons surveyed.

Is there a Magic Number?

Statisticians use various formulas to calculate sample size. The following chart provides a guideline to help determine the number of completed surveys needed for a representative sample with a 95% confidence level (plus or minus 5%). These numbers represent a good rule of thumb and can be used for defining sample size for community surveys where respondents fall into one of two subgroups and there is equal (50/50) probability of a respondent falling into one of the two subgroups (e.g., male/female).

However, you may want to survey more than two sub-groups, such as different school districts, age categories or income levels. To ensure a representative number of observations in a sub-group, determine the actual size of that subgroup and use Figure 1 to estimate a sample size for that subgroup.

Response Rates

Poor response rates are a serious problem in any survey. A needs assessment survey with a low return rate may not accurately represent the population. It is important to contact members of the original sample and request that they complete the survey. Use post card reminders, letters and phone calls to encourage responses. Send new surveys. The cost of reminder notes and letters must be incorporated into the cost of your survey a priori. One cannot compensate for a low return rate by starting with a larger sample or adding to the original sample once it becomes evident that the return rate is inadequate. A low response rate from a large sample is as unacceptable as a low response rate from a small sample.

For more information about community needs assessment surveys refer to the Laboratory for Community and Economic Development (LCED) publication, Conducting Needs Assessment Studies in Your Community, or ask your local unit office of the University of Illinois Cooperative Extension Service to contact LCED.

Other references:

• Babbie, E.R. (1990) Survey Research Methods. 2nd. Revised edition. Belmont, CA: Wadsworth Publishing Company.
• Dillman, D.A. (1978). Mail and Telephone Surveys: The Total Design Method. New York: John Wiley.
• Salant, P. And D.A. Dillman (1994). How to Conduct Your Own Survey. New York: John Wiley.