2.1 Introduction

In experimental studies, many aspects of the study design typically can be controlled by the researcher, so experimental studies are often easier to design to maximise internal validity. In contrast, observational studies have fewer design features that can be controlled by the researchers.

For example, treatments are not imposed in observational studies, so random allocation of treatments is impossible, and hence confounding is always a potential threat to internal validity in observational studies.

Nonetheless, researchers should still consider aspects of research design when designing observational studies, and manage those aspects when possible to maximise the internal validity. Specific design strategies that we consider for maximising internal validity are:

Not every design consideration will be relevant to every study.

Lemma 2.1 (Observational studies) Doll and Hill (1954) wrote to a large number of British doctors, and asked how much they smoked. Then they observed smokers and non-smokers for many years, and recorded who died of lung cancer.

Why is this is an observational study?
Because the value of the explanatory variable ('whether or not the doctor smoked') is not determined by the researcher. That would be unethical.


———. 1954. “The Mortality of Doctors in Relation to Their Smoking Habits.” British Medical Journal 1 (4877): 1451.