Reactivity is a phenomenon that occurs when individuals alter their performance or behavior due to the awareness that they are being observed.
It is a significant threat to a research study's external validity and is typically controlled for using blind experiment designs.
The Pygmalion effect occurs when students alter their behavior to meet teacher expectations.
[5][6] In addition there may be important individual differences in how participants react to a particular self-report measure.
Results gathered from unobtrusive methods tend to have very high test-retest reliability.