Each experiment involves a number of experimental units, which can be assigned at random (see below) to a treatment. The experimental unit should also be the unit of statistical analysis. It must be possible, in principle, to assign any two experimental units to different treatments. For this reason, if the treatment is given in the diet and all animals in the same cage therefore have the same diet, the cage of animals (not the individual animals within the cage) is the experimental unit. This situation can cause some problems. In studying the effects of an infection, for example, it may be necessary to house infected animals in one isolator and control animals in another. Strictly, the isolator is then the experimental unit because it was the entity assigned to the treatment and an analysis based on a comparison of individual infected versus noninfected animals would be valid only with the additional assumption (which should be explicitly stated) that animals within a single isolator are no more or no less alike than animals in different isolators. Although individual animals are often the experimental units assigned to the treatments, a crossover experimental design may involve assigning an animal to treatments X, Y, and Z sequentially in random order, in which case the experimental unit is the animal for a period of time. Similarly, if cells from an animal are cultured in a number of dishes that can be assigned to different in vitro treatments, then the dish of cells is the experimental unit. Split-plot experimental designs have more than one type of experimental unit. For example, cages each containing two mice could be assigned at random to a number of dietary treatments (so the cage is the experimental unit for comparing diets), and the mice within the cage may be given one of two vitamin treatments by injection (so the mice are experimental units for the vitamin effect). In each case, the analysis should reflect the way the randomization was done.