The extent to which there are discrepancies between survey results and the true value of what the survey researcher is attempting to measure. There are several possible sources of error here. Respondents may report inaccurate information because they do not have the required information, due to carelessness, or because they do not understand the question asked. Alternately, respondents may provide accurate information, but errors are introduced in the data processing stage due to keypunching, coding, or programming errors. Since it is often not possible to determine the "true value" of what one is trying to measure, precise estimates of measurement error are usually not possible. However, techniques exist for obtaining some information about the likely extent of measurement error. For example, information reported by individuals may be compared with appropriate institutional records on the individual.