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Random assignment of participants to experimental conditions is a commonly used experimental technique to help ensure that the treatment group and the control group are the same before treatment. For example, let us assume that we're curious to know the effects of eating an apple a day on your health (measured by blood pressure). One way of designing the study would be to select a sample of people and divide them into a control group (i.e., those who don't have an apple a day) and a treatment group (i.e., those who do have an apple a day).
How do you decide to divide your subjects? The best way is to do it randomly in order to cancel out the idiosyncrasies of your subject pool. Imagine if you decided to choose the groups based on cholesterol intake. You decide to have the low cholesterol group in the control group and the high cholesterol group in the treatment group. Would this bias the results of your study? Yes. Since cholesterol affects blood pressure, you as an experimenter would not know if the changes in health were due to the apple a day or the amount of cholesterol intake.