Editing Survey sampling (section)

==Bias in probability sampling==
{{Main|Sampling bias}}
Bias in surveys is undesirable, but often unavoidable.  The major types of bias that may occur in the sampling process are:
* [[Non-response bias]]:  When individuals or households selected in the survey sample cannot or will not complete the survey there is the potential for bias to result from this non-response.  Nonresponse bias occurs when the observed value deviates from the population parameter due to differences between respondents and nonrespondents.<ref>{{cite web|url=https://nces.ed.gov/StatProg/2002/glossary.asp|title=Glossary - NCES Statistical Standards|website=nces.ed.gov}}</ref>
* [[Response bias]]: This is not the opposite of non-response bias, but instead relates to a possible tendency of respondents to give inaccurate or untruthful answers for various reasons.
* Selection Bias: Selection bias occurs when some units have a differing probability of selection that is unaccounted for by the researcher.  For example, some households have multiple phone numbers making them more likely to be selected in a telephone survey than households with only one phone number.  This selection bias would be corrected by applying a [[survey weight]] equal to [1/(# of phone numbers)] to each household.
* [[Self-selection bias]]: A type of bias in which individuals voluntarily select themselves into a group, thereby potentially biasing the response of that group.
* [[Participation bias]]: Bias that arises due to the characteristics of those who choose to participate in a survey or poll.
* Coverage bias: Coverage bias can occur when population members do not appear in the sample frame (undercoverage).  Coverage bias occurs when the observed value deviates from the population parameter due to differences between covered and non-covered units.  Telephone surveys suffer from a well known source of coverage bias because they cannot include households without telephones.