A form of bias in clinical trials arising from systematic differences in the ways in which outcomes are defined and/or measured. For example, women taking an oral contraceptive will have more frequent cervical smears than women who are not on the pill and so are more likely to have cervical cancer diagnosed (if they actually have it). Thus, in a case-control study that compared women with cervical cancer and a control group, at least part of any higher pill consumption rates amongst the former group may be due to this effect. Also called 'ascertainment bias'.
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