The named ways a study can mislead
Selection bias arises when the people studied differ systematically from the target population, so the sample itself skews the result. Recall bias appears when cases remember exposures differently from controls, common in retrospective questionnaires. Confounding occurs when a third factor is linked to both the exposure and the outcome, creating a spurious association, as when age explains an apparent link between two unrelated things.
Each of these distorts the measured relationship in a characteristic direction rather than just adding random noise, which is what makes them dangerous: more data does not fix a biased design.
Lead-time and length-time bias in screening
Lead-time bias makes screen-detected patients appear to survive longer simply because diagnosis came earlier, even if the date of death is unchanged. Length-time bias arises because screening preferentially catches slow-growing disease, which has a better prognosis regardless of detection. Both can make screening look more effective than it is.
This is general educational information, not medical advice. Recognizing these biases helps in reading studies critically but does not substitute for guidance from qualified professionals.