There are certain sources of errors which occur both in sample surveys as well as in the complete enumeration. These errors are common in nature. Suppose we study each and every unit of the population. The population parameter under study is the population mean, and the ‘true’ value of the parameter is which is unknown. We hope to get the value of by a complete count of all the units of the population. We get a value called the ‘calculated’ or ‘observed’ value of the population mean. This observed value may be denoted by . The difference between and (true) is called a non-sampling error.
Even if we study the population units under ideal conditions, there may still be a difference between the observed value of the population mean and the true value of the population mean. Non-sampling errors may occur due to many reasons. Some of them are:
- The units of the population may not be defined properly. Suppose we have to carry out a study about the skilled labor force in our country. Who is a skilled person? Some people do more than one job. Some do the administrative jobs as well as the technical jobs. Some are skilled but they are working in an unskilled position. Thus it is important to clearly define the units of the population, otherwise there will be non-sampling errors both in the population count and the sample study.
- There may be a poor response on the part of respondents, and they do not supply correct information about their income, their children, their age and property, etc. These errors are likely to be of a high magnitude in population study than the sample study. To reduce these errors the respondents are to be persuaded.
- Data collection is subject to human error. The enumerators may be careless or they may not be able to maintain uniformity from place to place. The data may not be collected properly from the population or from the sample. These errors are likely to be more serious in the population data than the sample data.
- Another serious error is due to bias. Bias means an error on the part of the enumerator or the respondent when the data is being collected, and it may be intentional or unintentional. An enumerator may not be capable of reporting the correct data. If they have to report about the condition of crops in different areas after heavy rainfalls, their assessments may be biased due to lack of training or they may be inclined to give innacurate reports. Bias is a serious error and cannot be reduced by increasing the sample size. Bias may be present in the sample study as well as the population study.