Non-response is only one of a few things that can go wrong in a survey. There are many more areas of data collection and data processing that can introduce errors and thus affect the quality of results. The definitive result of all survey errors is a discrepancy between the survey estimate and the population characteristic to be estimated. This discrepancy is called the total survey error. Essentially two main categories can be pointed out contributing to this total error: sampling and nonsampling errors. Sampling errors are due to the sampling design. They are introduced when estimates are based on a sample and not on a total enumeration of the population. Such errors can in theory be avoided by a complete enumeration of the population. However, only a part of the population is used for estimating population characteristics. Nonsampling errors may occur even if the whole population is investigated. They denote errors made during the process of obtaining answers to questions asked. Nonsampling errors can arise from both observation and nonobservation errors. Non-observation errors are errors made when the intended measurements are not obtained. Undercoverage occurs when elements of the target population do not have a corresponding entry in the sampling frame. These population members cannot even be contacted. A type of non-observation error is non-response. This type of situation occurs when the sampled person does not provide the required information.