IweajunwaMBTM8106-8-5 5 If, however, participants are chosen from a pool that does not represent the study population, or from multiple pools with differing characteristics, they will neither be equivalent or probabilistically equivalent. Assigning subjects to control and experimental groups randomly is the best means of ensuring probabilistic equivalence at the beginning of a study, as long as the subjects are chosen from the appropriate population. Even though the groups will not be exactly the same, the probability that they are different can be calculated and minimized. There are also designs that allow for the use of the same participants in both control and experimental conditions, ensuring equivalence between conditions. Whether groups are probabilistically equivalent at the end of a study is the purpose of research. In a perfect world, where the treatment has an effect, the experimental group will consist of a different population from the control group at the end of a study, and this difference is the treatment effect. Unfortunately, there are confounds other than the application of the treatment that may exacerbate or minimize differences between the groups, consisting of the threats to internal validity. The best way to ensure that the only conditions that change between the groups is by designing the experiment to minimize threats to internal validity. By demonstrating that an independent variable precedes the dependent variables, and that they covary together, while showing that there are no other plausible explanations for the relationship, internal validity is high and can explain causality. With a properly designed study, the first two criteria are easy to control so it is critical to focus on the third criteria, and minimize alternative explanations.
Experiments and Quasi-Experiments
An experiment is a study in which the researcher manipulates the level of some independent variable and then measures the outcome. Experiments are powerful techniques for evaluating cause-and-effect relationships. Many researchers consider experiments the "gold standard" against which all other research designs should be judged. Experiments are conducted both in the laboratory and in real life situations.
Types of Experimental Design
There are two basic types of research design:
- True experiments
The purpose of both is to examine the cause of certain phenomena.
True experiments, in which all the important factors that might affect the phenomena of interest are completely controlled, are the preferred design. Often, however, it is not possible or practical to control all the key factors, so it becomes necessary to implement a quasi-experimental research design.
Similarities between true and quasi-experiments:
- Study participants are subjected to some type of treatment or condition
- Some outcome of interest is measured
- The researchers test whether differences in this outcome are related to the treatment
Differences between true experiments and quasi-experiments:
- In a true experiment, participants are randomly assigned to either the treatment or the control group, whereas they are not assigned randomly in a quasi-experiment
- In a quasi-experiment, the control and treatment groups differ not only in terms of the experimental treatment they receive, but also in other, often unknown or unknowable, ways. Thus, the researcher must try to statistically control for as many of these differences as possible
- Because control is lacking in quasi-experiments, there may be several "rival hypotheses" competing with the experimental manipulation as explanations for observed results
Key Components of Experimental Research Design
The Manipulation of Predictor Variables
In an experiment, the researcher manipulates the factor that is hypothesized to affect the outcome of interest. The factor that is being manipulated is typically referred to as the treatment or intervention. The researcher may manipulate whether research subjects receive a treatment (e.g., antidepressant medicine: yes or no) and the level of treatment (e.g., 50 mg, 75 mg, 100 mg, and 125 mg).
Suppose, for example, a group of researchers was interested in the causes of maternal employment. They might hypothesize that the provision of government-subsidized child care would promote such employment. They could then design an experiment in which some subjects would be provided the option of government-funded child care subsidies and others would not. The researchers might also manipulate the value of the child care subsidies in order to determine if higher subsidy values might result in different levels of maternal employment.
- Study participants are randomly assigned to different treatment groups
- All participants have the same chance of being in a given condition
- Participants are assigned to either the group that receives the treatment, known as the "experimental group" or "treatment group," or to the group which does not receive the treatment, referred to as the "control group"
- Random assignment neutralizes factors other than the independent and dependent variables, making it possible to directly infer cause and effect
Traditionally, experimental researchers have used convenience sampling to select study participants. However, as research methods have become more rigorous, and the problems with generalizing from a convenience sample to the larger population have become more apparent, experimental researchers are increasingly turning to random sampling. In experimental policy research studies, participants are often randomly selected from program administrative databases and randomly assigned to the control or treatment groups.
Validity of Results
The two types of validity of experiments are internal and external. It is often difficult to achieve both in social science research experiments.
- When an experiment is internally valid, we are certain that the independent variable (e.g., child care subsidies) caused the outcome of the study (e.g., maternal employment)
- When subjects are randomly assigned to treatment or control groups, we can assume that the independent variable caused the observed outcomes because the two groups should not have differed from one another at the start of the experiment
- For example, take the child care subsidy example above. Since research subjects were randomly assigned to the treatment (child care subsidies available) and control (no child care subsidies available) groups, the two groups should not have differed at the outset of the study. If, after the intervention, mothers in the treatment group were more likely to be working, we can assume that the availability of child care subsidies promoted maternal employment
One potential threat to internal validity in experiments occurs when participants either drop out of the study or refuse to participate in the study. If particular types of individuals drop out or refuse to participate more often than individuals with other characteristics, this is called differential attrition. For example, suppose an experiment was conducted to assess the effects of a new reading curriculum. If the new curriculum was so tough that many of the slowest readers dropped out of school, the school with the new curriculum would experience an increase in the average reading scores. The reason they experienced an increase in reading scores, however, is because the worst readers left the school, not because the new curriculum improved students' reading skills.
- External validity is also of particular concern in social science experiments
- It can be very difficult to generalize experimental results to groups that were not included in the study
- Studies that randomly select participants from the most diverse and representative populations are more likely to have external validity
- The use of random sampling techniques makes it easier to generalize the results of studies to other groups
For example, a research study shows that a new curriculum improved reading comprehension of third-grade children in Iowa. To assess the study's external validity, you would ask whether this new curriculum would also be effective with third graders in New York or with children in other elementary grades.
Glossary terms related to validity:
It is particularly important in experimental research to follow ethical guidelines. Protecting the health and safety of research subjects is imperative. In order to assure subject safety, all researchers should have their project reviewed by the Institutional Review Boards (IRBS). The National Institutes of Health supplies strict guidelines for project approval. Many of these guidelines are based on the Belmont Report (pdf).
The basic ethical principles:
- Respect for persons -- requires that research subjects are not coerced into participating in a study and requires the protection of research subjects who have diminished autonomy
- Beneficence -- requires that experiments do not harm research subjects, and that researchers minimize the risks for subjects while maximizing the benefits for them
- Justice -- requires that all forms of differential treatment among research subjects be justified
Advantages and Disadvantages of Experimental Design
The environment in which the research takes place can often be carefully controlled. Consequently, it is easier to estimate the true effect of the variable of interest on the outcome of interest.
It is often difficult to assure the external validity of the experiment, due to the frequently nonrandom selection processes and the artificial nature of the experimental context.