Observational study

In statistics, an observational study draws inferences about the 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 controlled experiments, such as randomized controlled trials, where each subject is randomly assigned to a treated group or a control group before the start of the treatment.

Reasons for uncontrolled experimentation
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 with a large subject pool of pregnant women and divide them randomly into a treatment group (receiving induced abortions) and a control group (bearing children), and then conduct regular cancer screenings for women from both groups.  Needless to say, such an experiment would run counter to common ethical principles.  (It would also suffer from various confounds and sources of bias, e.g., it would be impossible to conduct it as a blind experiment.)  The published studies investigating the abortion–breast cancer hypothesis generally start with a group of women who already have received abortions. Membership in this "treated" group is not controlled by the investigator: the group is formed after the "treatment" has been assigned.


 * The investigator may simply lack the requisite influence. Suppose a scientist wants to study the public health effects of a community-wide ban on smoking in public indoor areas.  In a controlled experiment, the investigator would randomly pick a set of communities to be in the treatment group.  However, it is typically up to each community and/or its legislature to enact a smoking ban.  The investigator can be expected to lack the political power to cause precisely those communities in the randomly selected treatment group to pass a smoking ban.  In an observational study, the investigator would typically start with a treatment group consisting of those communities where a smoking ban is already in effect.


 * A randomized experiment may be impractical. Suppose a researcher wants to study the suspected link between a certain medication and a very rare group of symptoms arising as a side effect.  Setting aside any ethical considerations, a randomized experiment would be impractical because of the rarity of the effect.  There may not be a subject pool large enough for the symptoms to be observed in at least one treated subject.  An observational study would typically start with a group of symptomatic subjects and work backwards to find those who were given the medication and later developed the symptoms. Thus a subset of the treated group was determined based on the presence of symptoms, instead of by random assignment.

Discussion
In all of those cases, if a randomized experiment cannot be carried out, the alternative line of investigation suffers from the problem that the decision of which subjects receive the treatment is not entirely random and thus is a potential source of bias. 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.

An observer of an uncontrolled experiment (or process) records potential factors and the data output: the goal is to determine the effects of the factors. Sometimes the recorded factors may not be directly causing the differences in the output. There may be more important factors which were not recorded but are, in fact, causal. Also, recorded or unrecorded factors may be correlated which may yield incorrect conclusions.

In observational studies, investigators may use propensity score matching (PSM) in order to reduce overt biases.

In 2007, several prominent medical researchers issued the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) statement, in which they called for observational studies to conform to 22 criteria that would make their conclusions easier to understand and generalise.