8.3 Other design considerations

Many issues must be kept in mind when designing studies, such as selecting the sample and managing confounding. In this section, some specific design considerations are discussed:

Not every design consideration will be relevant to every study.

8.3.1 Carry-over effect

Since treatments are not allocated in observational studies, carry-over effects may be present in some observational studies. It may be possible, however, to observe individuals who are exposed to Condition A then Condition B, and other individuals who are exposed to Condition B and then Condition A.

8.3.2 Hawthorne effect

In observational studies, individuals may 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. 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 8.3 (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.

8.3.3 Placebo effect

Observational studies can still have a control group, but the individuals are not randomly allocated to the control group. For example, in the Doll & Hill smoking study (Doll and Hill 1950), two groups were being compared: non-smokers (the control group) and smokers. Subjects were not allocated to the groups, however, so confounding remains a possibility. Again, the groups in the study can be compared (Example 8.2) to see if the groups are different in other ways.

8.3.4 Observer effect

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.

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


Doll R, Hill AB. Smoking and carcinoma of the lung. British Medical Journal. 1950;221(ii):739–48.
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.
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.
Wu K-S, Lee SS-J, Chen J-K, Chen Y-S, Tsai H-C, Chen Y-J, et al. Identifying heterogeneity in the Hawthorne effect on hand hygiene observation: A cohort study of overtly and covertly observed results. BMC Infectious Diseases. BioMed Central; 2018;18(1):369.