9.6. Repeated measures ANOVA

In repeated measures ANOVA, subjects are measured more than once. This section focuses on a repeated measures ANOVA with one within-groups and one between-groups factor (a common design). We’ll take our example from the field of physiological ecology. Physiological ecologists study how the physiological and biochemical processes of living systems respond to variations in environmental factors (a crucial area of study given the realities of global warming). The CO2 dataset included in the base installation contains the results of a study of cold tolerance in Northern and Southern plants of the grass species Echinochloa crus-galli (Potvin, Lechowicz, & Tardif, 1990). The photosynthetic rates of chilled plants were compared with the photosynthetic rates of nonchilled plants at several ambient CO2 concentrations. Half the plants were from Quebec and half were from Mississippi.

In this example, we’ll focus on chilled plants. The dependent variable is carbon dioxide uptake (uptake) in ml/L, and the independent variables are Type (Quebec versus Mississippi) and ambient CO2 concentration (conc) with seven levels (ranging from 95 to 1000 umol/m^2 sec). Type is a between-groups factor and conc is a within-groups factor. The analysis is presented in the next listing.