Sampling frame (synonyms: "sample frame", "survey frame") is the actual set of units from which a sample has been drawn: in the case of a simple random sample, all units from the sampling frame have an equal chance to be drawn and to occur in the sample. In the ideal case, the sampling frame should coincide with the population of interest.
Consider, for example, a survey aimed at establishing the number of potential customers for a new service in the population of New York City. The research team has drawn 1000 numbers at random from a telephone directory for the city, made 200 calls each day from Monday to Friday from 8am to 5pm and asked some questions.
In this example, population of interest is all inhabitants of the city; the sampling frame includes only those New Your City dwellers who satisfy all the following conditions:
- has a telephone;
- the telephone number is included in the directory;
- likely to be at home from 8am to 5pm from Monday to Friday;
- not a person who refuses to answer all telephone surveys.
The sampling frame in this case definitely differs from the population . For example, it under-represents the categories which either have no a telephone (e.g. the most poor), have an unlisted number, and who were not at home at the time of calls (e.g. employed people), who don´t like to participate in telephone interviews (e.g. more busy and active people). Such differences between the sampling frame and the population of interest is a main cause of bias in survey s and other methods aimed at random sampling .