Table 1 Important sources of bias independent of study design
Intervention characteristic | Potential bias & other influences on effect estimates |
|---|---|
Only a small proportion of the eligible IB population access the intervention | Selection - it is very difficult to get a comparison group with the same characteristics as the intervention group |
Intervention is to promote uptake of a service already available elsewhere (Jobcentre Plus) | Dilution - variation in exposure to the intervention across the study sample - risk of contamination among comparison group |
Intervention is targeted at socio-economically deprived population | Selection & Attrition - less likely to participate in the study - little incentive for comparison group to take part in research, withdrawals likely |
GP practices volunteer to participate in the intervention | Selection - primary care service that is already motivated to promote service use among vulnerable group Generalisability - GP practices who volunteer to take part may differ from those who do not |
Referral is opportunistic, referral criteria not well defined | Selection - referral decisions may vary within and between GPs |
Number of potential sample within an intervention practice is unknown | Selection - unknown number of eligible IB recipients will not be referred by GP |
Identification and referral of eligible IB recipients initiates the intervention, before recruitment to the evaluation study | Recall - Pre-intervention data reliant on medical notes and retrospective recall from IB recipient |
Expected short term health effects likely to be small | Study powered to detect small differences in health requires unfeasibly large population. Underpowered study may produce false result. |