Measurement error occurs when we do not measure the true value for a unit, but instead measure the true value plus an error. Measurement error is unavoidable in surveys, but it can be reduced.

The difference between a measured value and a true value.

The difference between the observed value and the measurand.

the difference between the true or actual state and that which is reported from measurements.

Error caused by the type or presentation of questions. In terms of online questionnaires, this is indicated when responses to the same question vary if the questionnaire is administered online or onsite.

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.

The difference between the measured value of a single item and its true value

Errors in measuring an explanatory variable in a regression that leads to biases in estimated parameters.