8.5 Observer effect and blinding researchers

The observer effect can be an issue in observational as well as experimental studies. For example, consider a study where the blood pressure of smokers and non-smokers is recorded (Verdecchia et al. 1995).

This is an observational study (individuals cannot be allocated to be a smoker or non-smoker), but if the researchers know whether or not the individual is a smoker when they record the blood pressure, then the observer effect could still come into play (recalling that the observer effect is an unconscious effect).

In this example, the observer effect could be managed if the researchers first measured the blood pressure, and then asked if the individual was a smoker or not. That is, the researchers may be able to be blinded to whether or not the subject is a smoker.

This may only be partially successful; the researcher may see the subject carrying a packet of cigarettes, or can smell smoke on their breath, for example; nonetheless, it may prove at least partially successful, and is easy to implement.

Example 8.6 (Blinding in ecology) A study of research articles in ecology found:

Across all 492 EEB articles surveyed, we judged 50.4% (\(n = 248\)) to have potential for observer bias, but only 13.3% (\(n = 33\) of \(248\)) of these articles stated use of blind observation.

Some articles explicitly stated the use of blind observation in the methods (\(n = 24\)), while others indicated indirectly that experiments had been done blind (\(n = 9\); e.g., use of a naïve experimenter…).

Kardish et al. (2015); line breaks added

Blinding the observer is not always possible, but should be used when possible to improve the internal validity of the study.

A study (Gamble and Walker 2016) found that bicycle riders who wear helmets are more likely to take risks compared to bicycle riders who do not wear helmets.

The paper states that the bicycle riders were blinded to the purpose of the study (reducing the impact of the Hawthorne effect), though clearly the participants knew they were involved in a study (so the impact was not completely eliminated).

However, the study was criticised (Radun and Lajunen 2018), since it was possible that

… the experimenters unconsciously conveyed their expectations to participants and thereby affected their responses […] it is clear that the double-blind procedure has been developed for a reason and should have been used in this study.

Radun and Lajunen (2018), p. 1020

The lack of blinding compromised the study’s internal validity.


Gamble T, Walker I. Wearing a bicycle helmet can increase risk taking and sensation seeking in adults. Psychological Science. Sage Publications Sage CA: Los Angeles, CA; 2016;27(2):289–94.
Kardish MR, Mueller UG, Amador-Vargas S, Dietrich EI, Ma R, Barrett B, et al. Blind trust in unblinded observation in ecology, evolution, and behavior. Frontiers in Ecology and Evolution. Frontiers; 2015;3:51.
Radun I, Lajunen T. Bicycle helmets and the experimenter effect. Psychological Science. SAGE Publications Sage CA: Los Angeles, CA; 2018;29(6):1020–2.
Verdecchia P, SchillacI G, Borgioni C, Ciucci A, Zampi I, Battistelli M, et al. Cigarette smoking, ambulatory blood pressure and cardiac hypertrophy in essential hypertension. Journal of Hypertension. 1995;13(10):1209–15.