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SAMPLINGThe source of one's data must be carefully selected. If collecting data directly from people, this is called your sample. A poor sample will result in poor quality data. Sometimes one is limited in sample selection and so one must always describe how the sample was selected in the evaluation findings. PopulationThe population is a group of individuals, objects, or items having a common characteristic. It is the total group from which a sample is selected. Examples of a population might be all citizens in a county, all foodservice establishments in a county, or all children registered in an afterschool food safety program. SamplingA sample is a subset of the population. A good sample is a smaller version of the population. The best sample is representative of the population. A sample is representative if important characteristics, such as age, educational level, ethnicity, income, are distributed similarly in both the sample and the population. For example, if one is collecting data about food safety practices in foodservice establishments in County A, the population is all foodservice establishments in County A. Most of the time one cannot access the entire population. In some counties, there are too many foodservice establishments, so it would not be feasible or even possible to study all. It would cost too much money, time, and effort. Therefore, a sample must be selected. Sampling is when one selects a smaller number (foodservice establishments) from the population (all foodservice establishments in the county) so an estimate can be made about the population (all foodservice establishments in the county). A properly selected sample can result in data that is accurate and precise, whereas one that is not properly selected will yield poor results. Selecting the SampleThere are two ways to select a sample – random sampling and purposeful sampling. Each is described below. Random sampling. The sample is randomly selected from the population. Every member of the target population can be included in the sample. It eliminates subjectivity when choosing a sample. Random sampling allows one to say that a sample is representative of the target population. Following are frequently used random sampling methods.
Purposeful sampling. This type of sample is chosen based on the characteristics of the target population and the need of the evaluation. Some members of the target population might have a greater chance of being chosen than do others. A purposeful sample does not rely on random selection of units so findings are not viewed as representative. One might have to choose a purposeful sample because an accurate list of the population might not be available, resources are limited, or obtaining cooperation from potential respondents is perceived to be difficult. A purposeful sample may be chosen to be sure to include a wide variety of people based on a number of critical characteristics. Sometimes, individuals are specifically chosen to represent a specific characteristic. More frequently, this type of sampling is used because it can be conveniently assembled. Following are examples of purposeful samples:
Determining the Sample SizeSampling error is large when the sample size is small. Therefore, it is best to use as large a sample as possible. The following table may be used to determine sample size base on a 5% error rate.
If the target population that is being evaluated has a total of 175 members, one needs to collect data from 122 members (175 is the population size and 122 is the sample size). If the target population has 10,000 members, one needs to collect data from 385 members. As the size of the population increases, it becomes easier to use a random sampling method because there are more individuals to choose from. Test Your Knowledge1. What is the definition for population? 2. What is a sample? 3. What are two ways that a sample can be selected? 4. To assure that certain subgroups are represented in proportion to their numbers in the population what type of sampling method is used? 5. If there are 325 restaurants in your county, what should the sample size be? 


