Completely Randomized Designs

Abstract

Suppose we have 4 different diets which we want to compare. The diets are labeled Diet A, Diet B, Diet C, and Diet D. We are interested in how the diets affect the coagulation rates of rabbits. The coagulation rate is the time in seconds that it takes for a cut to stop bleeding. We have 16 rabbits available for the experiment, so we will use 4 on each diet. How should we use randomization to assign the rabbits to the four treatment groups? The 16 rabbits arrive and are placed in a large compound until you are ready to begin the experiment, at which time they will be transferred to cages. Possible Assignment Plans Method 1: We assume that rabbits will be caught "at random". Catch four rabbits and assign them to Diet A. Catch the next four rabbits and assign them to Diet B. Continue with Diets C and D. Since the rabbits were "caught at random", this would produce a completely randomized design. Analyze the results as a completely randomized design. Method 1 is faulty. The first rabbits caught could be the slowest and weakest rabbits, those least able to escape capture. This would bias the results. If the experimental results came out to the disadvantage of Diet A, there would be no way to determine if the results were a consequence of Diet A or the fact that the weakest rabbits were placed on that diet by our "randomization process".