2.3 Carry-over effect and washout periods
The carry-over effect is a possible compromise to internal validity in observational studies. However, since treatments are not allocated in observational studies, carry-over effects may be difficult to prevent.
It may be possible, however, to observe individuals who are exposed to Condition A then Condition B, and other individuals who are exposed to Condition B and then Condition A.
Example 2.4 (Carry-over effects) A study of the carry-over effect in ecological observational studies gave many examples, including:
individuals occupying poor quality winter habitat may experience reduced reproductive success the following breeding season when compared to individuals occupying high quality winter habitat.
References
Norris, D. Ryan. 2005. “Carry-over Effects and Habitat Quality in Migratory Populations.” Oikos 109 (1): 178–86.