The term refers to what happens when the subjects of observational research deviate from their typical behavior because they realize that they are under observation.

I believe the answer is Hawthorne effect
When the subjects realize that they're under observation, they would made an effort to be included in the 'good' part of the data.
This will make them deviate from the course of actions/thought that they would usually have and would create a falsehood in data collection.

They went to leyden, holland

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