2.4 Hawthorne effect and blinding individuals

In observational studies, individuals may or may not know they are being observed. For example, in an observational study where subjects' blood pressure is measured (Verdecchia et al. 1995), subjects clearly will know that they are being observed. This has the potential to alter the data being recorded, and hence compromise internal validity.

As with experimental studies, efforts should be made to ensure that individuals do not know that they are being observed (that is, that the participants are blinded).

Example 2.5 (Hawthorne effect) One study (Wu et al. 2018) examined hand hygiene (HH) in a tertiary teaching hospital, using covert observers (that is, the observers were not obviously watching the hand hygiene practices of staff) and overt observers (that is, the observers were obvious about watching the hand hygiene practices of staff). One conclusions was that

The overall HH compliance was higher with overt observation than with covert observation (78% vs. 55%)...

--- Wu et al. (2018), p. 369

In other words, people's behaviour changed markedly when people knew they were being observed. This could easily change the observed relationship between the response and explanatory variables, and hence compromise internal validity.


Verdecchia, Paolo, Giuseppe SchillacI, Claudia Borgioni, Antonella Ciucci, Ivano Zampi, Massimo Battistelli, Roberto Gattobigio, Nicola Sacchi, and Carlo Porcellati. 1995. “Cigarette Smoking, Ambulatory Blood Pressure and Cardiac Hypertrophy in Essential Hypertension.” Journal of Hypertension 13 (10): 1209–15.
Wu, Kuan-Sheng, Susan Shin-Jung Lee, Jui-Kuang Chen, Yao-Shen Chen, Hung-Chin Tsai, Yueh-Ju Chen, Yu-Hsiu Huang, and Huey-Shyan Lin. 2018. “Identifying Heterogeneity in the Hawthorne Effect on Hand Hygiene Observation: A Cohort Study of Overtly and Covertly Observed Results.” BMC Infectious Diseases 18 (1): 369.