Read over your research objectives, including the research question. Determine which kinds of informants will need to be interviewed in order to fully address your research question. For example, if you're dealing with "experiences of health care workers in inner city hospitals," you'll need to speak with doctors, nurses, psychologists, specialists, radiologists and clerks. If you aren't sure what the relevant categories of people are in a given field, ask an administrative professional in the field.
Calculate the percentage that each informant group makes up of the general population. For example, if you need a sample of the general population of the United States, and you need to make sure that the ethnic breakdown has representative numbers of each racial group, you would need 12 percent African-American, 75 percent Caucasian, 3.6 percent Asian and 12.5 percent hispanic. If you are not sure where to find these percentages, visit the U.S. census website for basic information. If you are dealing with a more local population, visit the department or organization within the state or local government responsible for demographic information.
Calculate the number of people you will need in order to obtain a statistically significant sample. The size of your sample will be determined by the size of the population you are sampling. Find the size of your population from a demographic or professional reference book. For example, if you are sampling the U.S. population, refer to the census website. Once you have the size of the total population, enter this population into a sample size calculator. The calculator will give you an appropriate sample size.
Compute the number of people you will need from each informant category. Use the demographic percentages you found in step 2 to calculate the numbers needed. For example, if you require a total sample of 300, and you have a subgroup that makes up 12 percent of the larger population, multiply 300 by 0.12 (on your calculator, type in "300 x .12"), giving you a final figure of 36. In this case, 36 is the number of informants you will need from the demographic subcategory representing 12 percent of the population.
Make adjustments for changes in the size of the population. For example, if you are doing a long term study that requires a sample from each ethnic group in the population, and one ethnic group dramatically increases in population over the course of the year, re-do the calculations from steps 1 to 4 and find the amount of required informants.