When a quasi experimental design would be preferable to a between-subjects design
This page includes an explanation of the types, key components, validity, ethics, and advantages and disadvantages of experimental design. Show
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 DesignThere are two basic types of research design:
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:
Differences between true experiments and quasi-experiments:
Key Components of Experimental Research DesignThe Manipulation of Predictor VariablesIn 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. Random Assignment
Random SamplingTraditionally, 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 ResultsThe two types of validity of experiments are internal and external. It is often difficult to achieve both in social science research experiments. Internal Validity
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
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:
EthicsIt 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:
Advantages and Disadvantages of Experimental DesignAdvantagesThe 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. DisadvantagesIt 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. When would a quasiQuasi-experimental research designs do not randomly assign participants to treatment or control groups for comparison. Quasi-experimental designs are often used in educational research since students and teachers are not typically able to be randomly assigned to districts, schools, or classrooms for research purposes.
When might a quasiQuasi-experimental design is most useful in situations where it would be unethical or impractical to run a true experiment.
When would a quasiD (When full experimental control is not possible, it is appropriate for the researcher to select a quasi-experimental design.) 18.
Why would you use a quasiBenefits of quasi-experiments include: they can mimic an experiment and provide a high level of evidence without randomisation. there are several designs to choose from that you can adapt depending on your context. they can be used when there are practical or ethical reasons why participants can't be randomised.
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