Last time I heard the Hawthorne effect discussed (on BBC radio 4), two psychologists explained that the lighting changes were enacted on a Monday, and lasted for the week. The increase in productivity detected on the Monday was then attributed to the change in lighting from the previous Friday.

Despite this seeming like a classic case of conflating correlation with causation, and the workers simply being more productive due to being more rested.

The psychologists then concluded that although this wasn’t evidence for the Hawthorne effect, the effect was still real. I turned off the radio.