8.3 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 8.4 (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.

Norris (2005), p. 181


Norris DR. Carry-over effects and habitat quality in migratory populations. Oikos. Wiley Online Library; 2005;109(1):178–86.