Assignment: manipulation of treatments

Assignment: manipulation of treatments
Assignment: manipulation of treatments
Assignment: manipulation of treatments
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Assignment: The distinction between experimental and nonexperimental research
n The distinction between experimental and nonexperimental research rests on the manipulation of treatments and on random assignment.
n Any quantitative study without manipulation of treatments or random assignment is a nonexperimental study.
n Nonexperimental research is used when variables of interest cannot be manipulated because they are naturally existing attributes or when random assignment of individuals to a given treatment condition would be unethical.
n Numbers are used to represent different amounts of quantitative variables and different classifications of categorical variables.
n Nonexperimental studies may be classified along two dimensions: one based on the purpose of the study and the other on the time frame of the data collection. n Evidence of a relationship is not convincing evidence of causality.
n Alternative explanations for results in nonexperimental research should be explored and ruled out.
NOTE: My thanks to Professor Bill Frakes, from the Computer Science Department at Virginia Tech, and to students, including many from my Research Methods class in Fall 2007, for reviewing a prior draft of this chapter. Their insightful comments and suggestions helped improve this version. I take responsibility for any remaining elements of confusion that may remain.
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OVERVIEW OF NONEXPERIMENTAL RESEARCH
QUANTITATIVE RESEARCH is empirical, using numeric and quantifiable data. Conclusions are based on experimentation and on objective and systematic observations. Quantitative research may be divided into two general categories: experimental and nonexperimental. The essential elements of experimental research, which was discussed in detail in the previous chapter, are presented here first as a contrast to nonexperimental research. A primary goal for experimental research is to provide strong evidence for cause-and-effect relationships. This is done by demonstrating that manipulations of at least one variable, called the treatment or independent variable (IV), produce different outcomes in another variable, called the dependent variable (DV). An experimental study involves at least one IV that is manipulated or controlled by the researcher, random assignment to different treatment conditions, and the measurement of some DV after treatments are applied. Any resulting differences in the DV across the treatment groups can then be attributed to the differences in the treatment conditions that were applied.
In contrast to experimental research, nonexperimental research involves variables that are not manipulated by the researcher and instead are studied as they exist. One reason for using nonexperimental research is that many variables of interest in social science cannot be manipulated because they are attribute variables, such as gender, socioeconomic status, learning style, or any other personal characteristic or trait. For example, a researcher cannot randomly place individuals into different groups based on gender or learning style because these are naturally existing attributes.
Another reason to use nonexperimental research is that, in some cases, it would be unethical to randomly assign individuals to different treatment conditions. A classic example of this is that one could not study the effects of smoking by randomly assigning individuals to either a smoking or a nonsmoking group for a given number of years. The only ethical way to investigate the potential effects of smoking would be to identify a group of smokers and a group of nonsmokers and compare them for differences in their current state of health. The researcher, however, would also need to take other variables into account, such as how long people had smoked, their gender, age, and general health level. To do so would be important because the researcher cannot take for granted that the groups are comparable in aspects other than smoking behavior. This is in contrast to experimental groups, which, due to the process of random assignment, start out equal in all respects except for the treatment condition in which they are placed. In nonexperimental research, groups based on different traits or on self-selection, such as being or not being a smoker, may differ for any number of reasons other than the variable under investigation. Therefore, in nonexperimental studies, one cannot be as certain as in experimental studies that outcome differences are due to the independent variable under investigation. The researcher needs to consider possible alternative explanations, to jointly analyze several variables, and to present conclusions without making definitive causal statements.
Variables and Their Measurement 61
In this chapter, you will learn how to characterize nonexperimental studies that do not rely on either manipulation of variables or random assignment of subjects to groups. Different types of nonexperimental studies will be explained, and you will learn how to characterize them using a two-dimensional classification system. By the end of the chapter, you will understand the basic elements of nonexperimental studies, as well as the rationale for their use. Nonexperimental research examples, including published studies, will be incorporated into the discussion to facilitate understanding. At the end of the chapter, text and Web resources are provided to help you locate supplemental materials and additional information.