Nonsampling or Systematic Errors

Nonsampling errors can occur both in a sample survey and in a census. Such errors occur because of human mistakes and not chance. The errors that occur in the collection, recording, and tabulation of data are called nonsampling errors. Nonsampling errors occur because of human mistakes and not chance. Nonsampling errors can be minimized if questions are prepared carefully and data are handled cautiously. Many types of systematic errors or biases can occur in a survey, including selection error, nonresponse error, response error, and voluntary response error. The following chart shows the types of errors.
(i) Selection Error: When we need to select a sample, we use a list of elements from which we draw a sample, and this list usually does not include many members of the target population. Usually it is not feasible to include every member of the target population in this list. This list of members of the population that is used to select a sample is called the sampling frame. Thus, the sampling frame that is used to select a sample may not be representative of the population. This may cause the sample results to be different from the population results. The error that occurs because the sampling frame is not representative of the population is called the selection error. If a sample is nonrandom (and, hence, non representative), the sample results may be quite different from the census results.
(ii) Nonresponse Error: Even if our sampling frame and, consequently, the sample are representative of the population, nonresponse error may occur because many of the people included in the sample did not respond to the survey. This type of error occurs especially when a survey is conducted by mail. A lot of people do not return the questionnaires. It has been observed that families with low and high incomes do not respond to surveys by mail. Consequently, such surveys over represent middle-income families. This kind of error may also occur in a telephone survey. Many