Abstract: The same principles that apply to the design of experiments also apply to the design of an observational study, but instead of manipulating the animals, the observer controls the way he or she takes observations. The three principles are: independent replication; not confounding variables; and removing known sources of variation by blocking or matching. The chapter shows how these three principles can be applied to the design of observations, and discusses issues that often get in the way of valid designs such as pseudoreplication, experimenter bias, and choice of which animals to observe. Some simple observational designs are introduced.

Keywords: experiment design, independence, pseudoreplication, blocking, matching, randomization,