7.10 Summary
Designing effective experimental studies requires researchers to manage or minimise confounding where possible, by restricing the study to certain groups, by blocking, through special analysis methods, and/or through random allocation.
Well-designed experimental studies also try to manage the effects of the carry-over effect (for example, using a washout period, or randomly allocating treatments), the Hawthorne effect (for example, by blinding participants to the treatment), the observer effect (for example, by blinding the researchers to the treatments being applied), and the placebo effect (for example, by blinding participants to the treatments and by using controls).
The following short video may help explain some of these concepts: