The concept of reliability refers to the consistency with which the process of measurement takes place in a survey, experiment or other inquiry. It is closely related to the notion of repeatability – the extent to which, if the same field staff were to redo the survey, they would collect the same observation from each unit. Validity, on the other hand, refers to whether the answers obtained by asking a question on results of making a measurement are accurate for the sample (internal validity) and can be generalised to the wider population (external validity). Thus, the concept is closely linked to that of bias.