2.1 Observational Studies
2.1: Observational Studies
2.1.1: What are Observational Studies?
An observational study is one in which no variables can be manipulated or controlled by the investigator.
Learning Objectives
Identify situations in which observational studies are necessary and the challenges that arise in their interpretation.
Key Takeaways
Key Points
- An observational study is in contrast with experiments, such as randomized controlled trials, where each subject is randomly assigned to a treated group or a control group.
- Variables may be uncontrollable because 1) a randomized experiment would violate ethical standards, 2) the investigator may simply lack the requisite influence, or 3) a randomized experiment may be impractical.
- Observational studies can never identify causal relationships because even though two variables are related both might be caused by a third, unseen, variable.
- A major challenge in conducting observational studies is to draw inferences that are acceptably free from influences by overt biases, as well as to assess the influence of potential hidden biases.
- A major challenge in conducting observational studies is to draw inferences that are acceptably free from influences by overt biases, as well as to assess the influence of potential hidden biases.
Key Terms
- causality
- the relationship between an event (the cause) and a second event (the effect), where the second event is understood as a consequence of the first
- observational study
- a study drawing inferences about the possible effect of a treatment on subjects, where the assignment of subjects into a treated group versus a control group is outside the control of the investigator
A common goal in statistical research is to investigate causality, which is the relationship between an event (the cause) and a second event (the effect), where the second event is understood as a consequence of the first. There are two major types of causal statistical studies: experimental studies and observational studies. An observational study draws inferences about the possible effect of a treatment on subjects, where the assignment of subjects into a treated group versus a control group is outside the control of the investigator. This is in contrast with experiments, such as randomized controlled trials, where each subject is randomly assigned to a treated group or a control group. In other words, observational studies have no independent variables — nothing is manipulated by the experimenter. Rather, observations have the equivalent of two dependent variables.
In an observational study, the assignment of treatments may be beyond the control of the investigator for a variety of reasons:
- A randomized experiment would violate ethical standards: Suppose one wanted to investigate the abortion – breast cancer hypothesis, which postulates a causal link between induced abortion and the incidence of breast cancer. In a hypothetical controlled experiment, one would start wit