RANDOMIZATION

Introduction: Randomization involves placing subjects in groups at random. Random essentially means that every subject has an equal chance of being assigned to any group. If subjects are placed in groups randomly, there is no systematic bias in the groups with respect to attributes that could affect the dependent variable. Dependent variable

Let us consider the purpose of random assignment. Suppose we wanted to study the effectiveness of a contraceptive counseling program for multiparous women who have just given birth. Two groups of subjects are included one will be counseled and the other will not. The women in the sample are likely to differ from one another in many ways, such as age, marital status, financial situation, attitudes toward child-rearing, and others. Any of these characteristics could affect a woman’s diligence in practicing contraception, independent of whether she receives counseling. We need to have the “counsel” and “no counsel” groups equal with respect to these extraneous characteristics to assess the impact of the experimental counseling program on subsequent pregnancies. The random assignment of subjects to one group or the other is designed to perform this equalization function. One method might be to flip a coin for each woman. If the coin comes up “heads,” the woman would be assigned to one group; if the coin comes up “tails,” she would be assigned to the other group. Although randomization is the preferred scientific method for equalizing groups, there is no guarantee that the groups will, in fact, be equal. As an extreme example, suppose the study sample involves 10 women who have given birth to 4 or more children. Five of the 10 women are aged 35 years or older, and the remaining 5 are younger than age 35. We would expect random assignment to result in two or three women from the two age ranges in each group. But suppose that, by chance, the older five women all ended up in the experimental group. Because these women are nearing the end of their childbearing years, the likelihood of their conceiving is diminished. Thus, follow-up of their subsequent childbearing (the dependent variable) might suggest that the counseling program was effective in reducing subsequent pregnancies; however, a higher birth rate for the control group may reflect only age and fecundity differences, not lack of exposure to counseling.

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