Changes

==Selection bias==

==Selection bias==

Selection bias affects inclusion of individuals in the study and results in the study sample not being representative of the source population. This may occur in the initial selection process, or may be a result of nonresponse or losses to follow up during the study period. The mechanisms by which selection bias can enter a study will depend on the study design:

Selection bias affects inclusion of individuals in the study and results in the study sample not being representative of the source population. This may occur in the initial selection process, or may be a result of nonresponse or losses to follow-up during the study period. The mechanisms by which selection bias can enter a study will depend on the study design:

 

===Descriptive or cross sectional studies===

===Descriptive or cross sectional studies===

In these studies, lack of compliance may result in selection bias, as participation amongst selected individuals is very rarely 100% in these cases.  

In these studies, lack of compliance ('non-response') may result in selection bias, as participation amongst selected individuals is very rarely 100% in these cases. It is for this reason that compliance should be maximised as much as possible, through the use of reminders or incentives to participate.  

===Case-control studies===

===Case-control studies===

The main source of selection bias in case-control studies is through the selection of the '''control group'''. It is of vital importance that the control group comes from the same population as the case group (that is, if they happened to experience the outcome of interest during the study period, they would have been classified as a case instead of a control), and that there is no association between exposures of interest and selection as a control. The selection of the cases can also result in selection bias - in particular, in the case of hospital-based studies, where cases are selected from hospitals (as the population from which these individuals were drawn from makes selection of controls very difficult).

Case-control studies are particularly susceptible to selection bias. The main source of selection bias in these studies is through the selection of the '''control group'''. It is of vital importance that the control group comes from the same population as the case group (that is, if they happened to experience the outcome of interest during the study period, they would have been classified as a case instead of a control), and that there is no association between exposures of interest and selection as a control. The selection of the cases can also result in selection bias - in particular, in the case of hospital-based studies, where cases are selected from hospitals (as the population from which these individuals were drawn from makes selection of controls very difficult).

===Cohort studies===

===Cohort studies===

Selection bias in cohort studies is generally less likely than in other studies, as selection for participation in the study generally precedes the development of the outcome of interest. However, losses to follow-up provide a mechanism by which selection bias may be introduced. For example, in the case of a perceived 'risky behaviour', it is plausible that individuals engaged in this may be less likely to remain in the study, meaning that these individuals (which may be more likely to experience the outcome) will be lost from the study.

Selection bias in cohort studies is generally less likely than in case-control studies, as selection for participation in the study generally precedes the development of the outcome of interest. However, non-response may result in some biases, and losses to follow-up during the study may also introduce selection bias. For example, a study may be conducted in order to investigate the effect of farm biosecurity on introduction of disease. It is plausible that farmers with poor biosecurity will be less likely to remain in the study, meaning that these farms individuals (which may be more likely to experience disease problems) will be lost from the study.

==Information bias==

==Information bias==

Non-differential bias will tend to reduce the strength of any association present, and will increase the probability of a [[Random variation#Hypothesis testing and study power|type II error]], whereas the effect of differential bias cannot be predicted.

Non-differential bias will tend to reduce the strength of any association present, and will increase the probability of a [[Random variation#Hypothesis testing and study power|type II error]], whereas the effect of differential bias cannot be predicted.

[[Category:Veterinary Epidemiology - General Concepts|I]]

[[Category:Veterinary Epidemiology - General Concepts|I]]