An effect is a difference between what happened and what would have happened otherwise. A study with no control group has measured only the first, and the second is supplied by assumption rather than observation.
An effect is a difference between what happened and what would have happened otherwise. A study with no control group has measured only the first, and the second is supplied by assumption rather than observation.
Many conditions fluctuate, and people typically seek intervention when they are at their worst. Measuring again later finds improvement whether or not anything was done, which is why uncontrolled before-and-after comparisons systematically overstate effects.
Extreme measurements tend to be followed by less extreme ones purely because extremes are partly chance. Selecting people for a study because their values are extreme guarantees apparent improvement at the next measurement.
Comparing against an existing treatment asks whether something is better than current practice. Comparing against placebo asks whether it does anything at all. Both are legitimate and they are not interchangeable, and the choice should match the claim.
Comparing against patients treated in the past confounds the intervention with everything else that changed. Diagnosis, supportive care and who gets included all shift over time, usually in the direction that flatters the newer approach.
People told they will receive something later behave differently from people receiving nothing, and the comparison includes that difference. It is often the only ethical option and the result should be read with the limitation attached.
Having each person receive both conditions controls for differences between people efficiently. It requires that effects wash out between periods, which is an assumption worth checking rather than accepting.
What the control group received, how people were assigned to it, and whether the two groups were measured the same way by the same people.
A control group supplies the comparison; blinding conceals who is in which. A trial can have one without the other, and the combination that most overstates effects is an unblinded comparison on a subjective outcome.
Participants receiving monitoring, contact and expectation may improve for those reasons alone. Controls designed to match that attention isolate the treatment from the experience of being treated.
The claims on this page about the comparison that turns a sequence of events into evidence can be checked against sources outside this site: