Changes

'''Systematic error''', or 'bias' is of particular importance in any epidemiological investigation, and should be avoided wherever possible. Biases will reduce the '''validity''' of any results obtained, whether it be by overestimating or underestimating the frequency of disease in a population or the association between an exposure and disease. The forms of bias covered here can only be minimised through careful study design and execution - they cannot be accounted for in the analysis. Although [[confounding]] is considered by many authors as a form of bias, it can be accounted for during analysis, and so is covered separately.<br>

'''Systematic error''', or 'bias' is of particular importance in any epidemiological investigation, and should be avoided wherever possible. Biases will reduce the '''validity''' of any results obtained, whether it be by overestimating or underestimating the frequency of disease in a population or the association between an exposure and disease. The forms of bias covered here can only be minimised through careful study design and execution - they cannot be accounted for in the analysis. Although [[confounding]] is considered by many authors as a form of bias, it can be accounted for during analysis, and so is covered separately.<br>

Bias can be introduced into a study through the selection of participants ('''selection bias'''), or through errors made in the classification of measurement of exposures/outcomes of interest ('''information bias'''). These biases may also be classified as '''differential''' or '''non-differential''', depending on whether the likelihood of bias is affected by an individual's exposure to factors of interest, or experience of the outcome of interest.

Bias can be introduced into a study through the selection of participants ('''selection bias'''), or through errors made in the classification of measurement of exposures/outcomes of interest ('''information bias''').  

==Validity==

==Validity==

As mentioned [[Sampling strategies#Populations and samples|earlier]], the '''validity''' of an estimate is a measure of how well it can be extrapolated to the population in question (or, in the case of ''external validity'', to other populations). The same concept applies to analytic studies, although the validity here relates to the ability to estimate the association between an exposure and an outcome of interest. Both selection and information biases can reduce the validity of an estimate.<br>

As mentioned [[Sampling strategies#Populations and samples|earlier]], the '''validity''' of an estimate of disease occurrence is a measure of how well it can be extrapolated to the source population (or, in the case of ''external validity'', to other populations). The same concept of validity applies to analytic studies, although in these cases it relates to the estimation of the association between an exposure and an outcome of interest. Both selection and information biases act to reduce the validity of an estimate.<br>

 

==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. As alluded to above, the process of bias introduction into descriptive analytic studies differs from that in analytic studies, due to different aims in these investigations. In descriptive studies, any process which results in the selection of a population which is not representative of the source population can bias the results of the study. However, in the case of analytic studies, the representation of the source population is less important, as long as the measure of association between the exposure and outcome is not affected. In these cases, if selection is associated with both exposure and outcome (for example, if 'diseased' individuals who were exposed to the factor of interest were underrepresented in the sample), then bias will result. Note, however, that if selection is 'only' associated with 'either' exposure status or outcome (for example, if diseased individuals in general were underrepresented, regardless of whether they were exposed to the factor or not), then bias will not result. These concepts are similar to the concept of '''differential''' and '''non-differential''' information bias described below.<br>

===Descriptive studies===

===Descriptive studies===

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

In descriptive studies, the precise definition of the source population and the use of an appropriate method of selection of individuals from this is of paramount importance in order to minimise bias. However, even when this is achieved effectively, a lack of compliance amongst selected individuals ('non-response') can introduce considerable bias. Increasing compliance, through the use of reminders or incentives to participate, can help reduce this source of error.  

   

   

===Cross sectional studies===

===Analytic studies===

Non-response is also the main route of selection bias entering these studies,

* non-response bias (if the association between exposure and outcome differs between responders and non-responders)

* inappropriate selection of comparison groups (both groups should be from the same source population)

 

However, individual study designs may be associated with other sources of selection bias:

===Case-control studies===

===Case-control studies===

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).

Case-control studies are particularly susceptible to selection bias. A major source of selection bias in these studies is through the selection of the '''control group'''. As mentioned above, 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). ''Admission risk bias'' is a type of selection bias associated with hospital-based studies where cases and controls are selected from hospitals (or similar establishments), and so may have a different exposure profile to the general population (to which the results are generally intended to be extrapolated). However, selection of controls from other sources can also introduce biases. Detection bias is a form of selection bias which results from differential classification of disease status according to exposure status (and so is closely related to '''information bias''').

===Cohort studies===

===Cohort studies===

==Information bias==

==Information bias==

Information bias results from errors in measurement or classification of exposures or outcomes of interest amongst the individuals included in the study. In the case of analytic studies, this may be classified as '''differential''' or '''non-differential'''. whereas the effect of differential bias cannot be predicted.<br>

Information bias results from errors in measurement or classification of exposures or outcomes of interest amongst the individuals included in the study. In the case of analytic studies, this may be classified as '''differential''' or '''non-differential''', depending on whether the likelihood of bias is affected by an individual's exposure to factors of interest, or experience of the outcome of interest.

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===Non-differental information bias/Measurement error===

===Non-differental information bias/Measurement error===