What happens if the control group differs from the treatment group?
Both groups are exposed to the same conditions except for the intervention: the treatment group receives the intervention, whereas the control group does not. The outcome is measured simultaneously for both groups at 2 points in time — the pretest and the posttest.
What is the difference between an intervention and a control group?
The experience of one of the groups in the study (e.g., history, maturation) may differ systematically from the other group. The differential influence of this threat among groups is referred to as an interaction with selection. Remedy: random assignment to treatment groups. Defining External Validity: Three Questions.
Why is a pretest-only design better than a control group design?
Experimental group- group given the treatment Control group- group not exposed to the stimulus or predictor variable ... -eliminates possible pretest reactivity by studying both an experimental and a control group after the experimental group has been exposed to some treatment ... -offer the best control for factors that tend to affect the ...
What happens when participants are not randomly assigned to conditions?
participants in one condition are exposed to a treatment participants in the other nonequivalent condition are not exposure to the treatment scores from both groups are obtained after the treatment ends addition of the control group may enhance the studies validity, but it may nothing to reduce potential confounding variables
Why should you include both a control and treatment group?
In a well-designed experiment, all variables apart from the treatment should be kept constant between the two groups. This means researchers can correctly measure the entire effect of the treatment without interference from confounding variables.Jul 3, 2020
What is treatment misidentification?
Treatment misidentification occurs when some process that the researcher is not aware of is responsible for the apparent effect of treatment.
What is an example of quasi-experimental design?
This is the most common type of quasi-experimental design. Example: Nonequivalent groups design You hypothesize that a new after-school program will lead to higher grades. You choose two similar groups of children who attend different schools, one of which implements the new program while the other does not.Jul 31, 2020
What are the threats to internal validity?
There are eight threats to internal validity: history, maturation, instrumentation, testing, selection bias, regression to the mean, social interaction and attrition.May 1, 2020
What do you call the group that is exposed to treatment in experimental research?
In a psychology experiment, the experimental group (or experimental condition) refers to the group of participants who are exposed to the independent variable. These participants receive or are exposed to the treatment variable.Apr 24, 2020
When conducting an experiment the control group receives the treatment and the experimental group does not?
One group is exposed to the intervention (the experimental group, also known as the treatment group) and the other is not exposed to the intervention (the control group). In some cases, it may be immoral to withhold treatment from a control group within an experiment.
Does a quasi-experiment have a control group?
"Quasi-experimental research is similar to experimental research in that there is manipulation of an independent variable. It differs from experimental research because either there is no control group, no random selection, no random assignment, and/or no active manipulation."Apr 8, 2022
What is pre experimental design?
Pre-experimental design is a research method that happens before the true experiment and determines how the researcher's intervention will affect the experiment.
What is the control group science?
The control group is composed of participants who do not receive the experimental treatment. When conducting an experiment, these people are randomly assigned to be in this group. They also closely resemble the participants who are in the experimental group or the individuals who receive the treatment.Oct 4, 2020
What is treatment diffusion?
a situation in which research participants adopt a different intervention from the one they were assigned because they believe the different intervention is more effective.
How can you control threats to internal validity?
Internal ValidityKeep an eye out for this if there are multiple observation/test points in your study.Go for consistency. Instrumentation threats can be reduced or eliminated by making every effort to maintain consistency at each observation point.Oct 16, 2015
What determines internal validity?
Internal validity depends largely on the procedures of a study and how rigorously it is performed. Internal validity is not a "yes or no" type of concept. Instead, we consider how confident we can be with the findings of a study, based on whether it avoids traps that may make the findings questionable.Jul 31, 2021
How to improve interrupted time series design?
One way to improve upon the interrupted time-series design is to add a control group. The interrupted time-series design with nonequivalent groups involves taking a set of measurements at intervals over a period of time both before and after an intervention of interest in two or more nonequivalent groups. Once again consider the manufacturing company that measures its workers’ productivity each week for a year before and after reducing work shifts from 10 hours to 8 hours. This design could be improved by locating another manufacturing company who does not plan to change their shift length and using them as a nonequivalent control group. If productivity increased rather quickly after the shortening of the work shifts in the treatment group but productivity remained consistent in the control group, then this provides better evidence for the effectiveness of the treatment.
How to improve posttest only nonequivalent groups design?
Another way to improve upon the posttest only nonequivalent groups design is to add a pretest. In the pretest-posttest nonequivalent groups design t here is a treatment group that is given a pretest, receives a treatment, and then is given a posttest. But at the same time there is a nonequivalent control group that is given a pretest, does not receive the treatment, and then is given a posttest. The question, then, is not simply whether participants who receive the treatment improve, but whether they improve more than participants who do not receive the treatment.
What is a posttest only nonequivalent group?
The first nonequivalent groups design we will consider is the posttest only nonequivalent groups design. In this design, participants in one group are exposed to a treatment, a nonequivalent group is not exposed to the treatment, and then the two groups are compared. Imagine, for example, a researcher who wants to evaluate a new method of teaching fractions to third graders. One way would be to conduct a study with a treatment group consisting of one class of third-grade students and a control group consisting of another class of third-grade students. This design would be a nonequivalent groups design because the students are not randomly assigned to classes by the researcher, which means there could be important differences between them. For example, the parents of higher achieving or more motivated students might have been more likely to request that their children be assigned to Ms. Williams’s class. Or the principal might have assigned the “troublemakers” to Mr. Jones’s class because he is a stronger disciplinarian. Of course, the teachers’ styles, and even the classroom environments might be very different and might cause different levels of achievement or motivation among the students. If at the end of the study there was a difference in the two classes’ knowledge of fractions, it might have been caused by the difference between the teaching methods—but it might have been caused by any of these confounding variables.
What are the three types of quasi-experimental designs?
These are the one-group posttest only design, the one-group pretest-posttest design, and the interrupted time-series design. There are five types of quasi-experimental designs that are between-subjects in nature.
What is a nonequivalent group design?
A nonequivalent groups design, then, is a between-subjects design in which participants have not been randomly assigned to conditions.
Why is a nonequivalent design considered a nonequivalent design?
This design would be a nonequivalent groups design because the students are not randomly assigned to classes by the researcher , which means there could be important differences between them.
What happens if students in a similar school are given a pretest?
Students in a similar school are given the pretest, not exposed to an anti-drug program, and finally, are given a posttest. Again, if students in the treatment condition become more negative toward drugs, this change in attitude could be an effect of the treatment, but it could also be a matter of history or maturation.
Advantages of the pretest-posttest control group design
By using a pretest, a control group, and random assignment, this design controls all internal threats to validity.
Limitations of the pretest-posttest control group design
Participants included in any randomized study might not be typical people in the population i.e. they may not represent well the population of interest, this is because:
Example of a study that used the pretest-posttest control group design
Koenig et al. used a pretest-posttest control group design to study the effect of a yoga program on the classroom behavior of autistic children. These children were randomly assigned to either receive the yoga program or their standard morning routine.
Abstract
Mainstream research design in the social and behavioral sciences has often been conceptualized using a taxonomy of threats to experimental validity first articulated by Campbell and his colleagues (Campbell & Stanley, 1966; Cook & Campbell, 1979 ).
Four Types of Validity
Shadish et al. ( 2002) define statistical conclusion validity as the validity of the conclusion that the dependent variable covaries with the independent variable, as well as that of any conclusions regarding the degree of their covariation.
Concluding Remarks
Some of the validity threats described by Shadish et al. ( 2002) appear to apply mainly to group-design experiments and/or experiments in which inferential statistics are employed for the purpose of data analysis.
Acknowledgments
We thank Molli Luke for her comments on an earlier version of the manuscript.
Footnotes
1 Shadish et al. ( 2002) discuss validity threats exclusively in the context of group-design experiments, whereas in applied behavior analysis, single-case experiments predominate. As the authors note, a single-case experimental design may be considered an instance of an interrupted time-series design applied to one individual.
Types
- Although control groups are more common in experimental research, they can be used in other types of research too. Researchers generally rely on non-experimental control groups in two cases: quasi-experimental or matching design.
Examples
Variations
- Recall that when participants in a between-subjects experiment are randomly assigned to conditions, the resulting groups are likely to be quite similar. In fact, researchers consider them to be equivalent. When participants are not randomly assigned to conditions, however, the resulting groups are likely to be dissimilar in some ways. For this reason, researchers consider them to b…
Advantages
- The first nonequivalent groups design we will consider is the posttest only nonequivalent groups design. In this design, participants in one group are exposed to a treatment, a nonequivalent group is not exposed to the treatment, and then the two groups are compared. Imagine, for example, a researcher who wants to evaluate a new method of teaching fractions to third graders. One way …
Treatment
- Some of these nonequivalent control group designs can be further improved by adding a switching replication. Using a pretest-posttest design with switching replication design, nonequivalent groups are administered a pretest of the dependent variable, then one group receives a treatment while a nonequivalent control group does not receive a treatment, the depe…
Significance
- One of the strengths of this design is that it includes a built in replication. In the example given, we would get evidence for the efficacy of the treatment in two different samples (patients and students). Another strength of this design is that it provides more control over history effects. It becomes rather unlikely that some outside event would perfectly coincide with the introduction …