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The Hawthorne effect matters because it is a major threat to the validityHow well a measurement or test actually measures the thing it is meant to measure.Read more → of clinical and workplace studies. It describes people changing their behaviour simply because they know they are being observed, not because of the actual intervention being tested. For example, imagine staff on a ward who wash their hands more often during an infection-control audit week. Once the audit ends, hand hygiene slips straight back down. The improvement was reactivity (a behaviour change caused by awareness of being watched), not a genuine change caused by any training given. It takes its name from productivity studies at the Hawthorne electric works, where output rose whenever workers knew they were being studied, regardless of what was actually changed.

Reactivity is the general term for behaviour changing because of the awareness of being observed or measured. In practice, researchers reduce the Hawthorne effect using control groups (a comparison group observed identically but not given the intervention) and, where possible, blindingKeeping participants and/or researchers unaware of who is receiving which treatment, to stop that knowledge from influencing results.Read more →. That way, any change caused purely by observation is measured and subtracted out, rather than mistaken for a true treatment effect. Take a study of a new ward round checklist showing improved outcomes on an observed intervention ward but with no proper control ward. The improvement could simply reflect staff behaving differently under observation, rather than any real effect of the checklist itself.

• The original studies that gave this effect its name were conducted at an industrial plant during the 1920s and 1930s. In which country was that plant located?
• A researcher argues the original findings have been overstated because of weaknesses in how the studies were run. Which criticism is best supported by later reanalysis?
• A quality improvement team measures infection rates before and after introducing a new checklist, and rates improve. Which threat to their conclusion does this effect represent?
• A trial team is told their study is at risk of both a Hawthorne effect and a demand characteristic. What distinguishes these two phenomena?

• Hawthorne effect, behaviour changes because of being observed, unlike the placeboAn inactive treatment given to a comparison group so any real effect of the active treatment can be judged against it.Read more → effect, where a genuine physiological or psychological response follows a believed-active but inert treatment.
• Hawthorne effect, applies to the observed participant, whereas observer-expectancy effect (Rosenthal) is the observer's own biasA systematic error that skews a study's results away from the truth in a particular direction.Read more → unconsciously influencing how they record results.
• Reactivity, a general threat to validityHow well a measurement or test actually measures the thing it is meant to measure.Read more → from being watched, rather than a specific intervention effect the study is trying to measure.

• The studies were not conducted in that country.
• The original work was not extensively replicated at the time.
• No evidence indicates the data were fabricated outright.
• Awareness of being studied is central to the effect described.
• Illumination was genuinely varied during the experiments.

• Name the learning, social or research principle being tested before reading the options.
• Identify the antecedent, behaviour, consequence or cognitive process in the vignette.
• Compare the direction of effect with the nearest competing principle.
• Apply the same rule to a new example to confirm that the answer transfers.