Sources of variation the most obvious is perhaps that

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Sources of Variation: The most obvious is, perhaps, that the rabbits are all different rabbits, and so they will all grow at different rates. If different breeds of rabbit are used, then we will have an additional source of variation. Young California rabbits do not grow at the same rate as young Florida White rabbits, for example. The environment in which the rabbits live will not be exactly the same. They cannot all live in the same location; some will be in slightly warmer areas while others will be in areas with more light. They will not all have exactly the same amount of exercise or sleep. The food will be carefully weighed before it is given to the rabbits, but there will inevitably be measurement error in the amount of food given to each rabbit. Similarly, the rabbits will be weighed before the experiment begins and after the experiment ends. Measurement error (hopefully small) will occur in both these weighings. All of the aspects of the experimental setting mentioned so far can be considered natural chance-like variation. There is another source of essential variation: the systematic difference in the rate of growth that is a result of the different diets. This is a variation we want to investigate. One way to think about the goal of the experiment is that we want to know if the variation that is a result of the diet is larger than the variation that is due to all the natural variation inherent in rabbit growth. In designing our experiment, we want to accentuate this planned, systematic variation, while reducing the natural chance-like variation. This chapter considers some of the methods the experimenter has for managing these three sources of variation in the example experiment. The methods are Control, Randomization, Replication, and Blocking.
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Chapter 5: Producing Data Control: We control the experiment by organizing the structural components of the experiment to remove as many sources of chance-like variation as possible. We want to keep the experimental units (in this example, the rabbits in their cage) as similar as possible. We would like to use only one breed of rabbit. We might also prefer to use just one gender of rabbit, since male and female growth patterns may differ. We certainly want to keep the cages in a single location so that the effects of heat, light, air flow, and other unknowable affects will be as consistent as possible. As much as possible, we want the only difference among the rabbits to be the diet they are receiving. To control the measurement error, we want to use the same scale when measuring the food each day. Similarly, it is important to use the same scales to measure the weights of the rabbits before and after the experimental regimen. We would prefer to use the same technician as well. In the end, no matter how much control we have in our experiment, some chance- like variation remains. It is the natural variation in average weight gain for our rabbits. By control, we try to make sure that
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