Designing effective observational studies requires researchers to maximise internal validity. This can be achieved by managing confounding where possible, as confounding is often a major threat to the internal validity of observational studies. This can be managed by restricing the study to certain groups, by blocking, or through special analysis methods. Random allocation is not possible in observational studies. For this reason, observing, measuring, assessing or recording all the information that is likely to be important for understanding the data is important, usually to be used in analysis.
Well-designed observational studies also try to manage the effects of the carry-over effect, the Hawthorne effect, the observer effect, and the placebo effect, though the means of doing so are often not under the control of the researchers.