What is posttest only control group?
The posttest-only control group design is a basic experimental design where participants get randomly assigned to either receive an intervention or not, and then the outcome of interest is measured only once after the intervention takes place in order to determine its effect.
When did Topf and Davis use a posttest-only control group design?
In 1993 , Topf and Davis used a posttest-only control group design to examine if CCU (Critical Care Unit) noise affects REM (Rapid Eye Movement) sleep.
Why is pretest unnecessary?
For instance, a pretest is unnecessary if the outcome we want to measure is mortality and all participants are alive at the start of the study.
How does a static group comparison design differ from a pretest posttest randomized controlled trial?
Note that this design differs from the pretest-posttest randomized controlled trial by having no measurements taken before the intervention. Also, if we remove the random assignment component from this design (and let participants get assigned to groups according to their choosing or that of the researcher), we get the static-group comparison design which is a type of quasi-experiment.
What is simultaneity in measuring both groups?
The simultaneity in measuring both groups controls factors that change with time (as these may also affect the outcome and bias the study).
What is the use of 1 measurement?
The use of 1 measurement only controls factors related to the instruments used to measure the outcome (since the device is only used once to measure the outcome, we won’t have to deal with changes that can happen to the device or to the quality of measurements from 1 measurement to the next).
Why is the initial difference in the characteristics of both groups important?
This is an advantage over static-group comparison where assignment is not done at random, so the initial difference in the characteristics of both groups can be an important factor influencing the outcome of the study and therefore an important source of bias. 2. External factors are controlled.
What is a one group posttest only design?
In a one-group posttest only design, a treatment is implemented (or an independent variable is manipulated) and then a dependent variable is measured once after the treatment is implemented. Imagine, for example, a researcher who is interested in the effectiveness of an anti-drug education program on elementary school students’ attitudes toward illegal drugs. The researcher could implement the anti-drug program, and then immediately after the program ends, the researcher could measure students’ attitudes toward illegal drugs.
What is the threat to the internal validity of one group pretest-posttest designs?
Another threat to the internal validity of one-group pretest-posttest designs is testing which refers to when the act of measuring the dependent variable during the pretest affects participants’ responses at posttest. For instance, completing the measure of attitudes towards illegal drugs may have had an effect on those attitudes. Simply completing this measure may have inspired further thinking and conversations about illegal drugs that then produced a change in posttest scores.
Why did participants change between the pretest and the posttest?
Participants might have changed between the pretest and the posttest in ways that they were going to anyway because they are growing and learning. If it were a year long anti-drug program, participants might become less impulsive or better reasoners and this might be responsible for the change in their attitudes toward illegal drugs.
What is pretest post test?
The pretest-posttest design is much like a within-subjects experiment in which each participant is tested first under the control condition and then under the treatment condition. It is unlike a within-subjects experiment, however, in that the order of conditions is not counterbalanced because it typically is not possible for a participant ...
What are the limitations of quasi-experimental design?
This is the weakest type of quasi-experimental design. A major limitation to this design is the lack of a control or comparison group. There is no way to determine what the attitudes of these students would have been if they hadn’t completed the anti-drug program. Despite this major limitation, results from this design are frequently reported in the media and are often misinterpreted by the general population. For instance, advertisers might claim that 80% of women noticed their skin looked bright after using Brand X cleanser for a month. If there is no comparison group, then this statistic means little to nothing.
Is the average posttest score better than the average pretest score?
If the average posttest score is better than the average pretest score (e.g., attitudes toward illegal drugs are more negative after the anti-drug educational program), then it makes sense to conclude that the treatment might be responsible for the improvement.
Advantages of The Posttest-Only Control Group Design
- 1. The treatment and control groups are equivalent at baseline
This is very important because when the 2 groups are equal, any difference in the outcome measured will be attributed only to the intervention and not to the initial difference between the groups. Note that these groups are not expected to be perfectly equal, in fact they don’t need to … - 2. External factors are controlled
1. The use of a control group controls history(i.e. adjusts for the effects of events that can happen at the same time as the intervention and can influence the outcome, therefore becoming a rival hypothesis and a potential source of bias). 2. The use of 1 measurement only controls factors re…
Limitations of The Posttest-Only Control Group Design
- 1. High risk of attrition bias
This is due to participants quitting the study for different reasons between the study groups making the groups unequal anymore. Note that the risk of attrition bias is higher when the intervention takes a long period of time to be implemented (in the order of days or weeks). The a… - 2. The effect of the intervention on subgroups is not clear
In this posttest-only design we cannot compare the outcome with pretest measures, meaning that we cannot investigate which subgroup of participants responded more to the treatment or which subgroup did not respond well.
Example of A Study That Used A Posttest-Only Control Group Design
- In 1993, Topf and Davisused a posttest-only control group design to examine if CCU (Critical Care Unit) noise affects REM (Rapid Eye Movement) sleep. So they randomly assigned 70 women with no hearing or sleeping problems to attempt to sleep in one of the following conditions: 1. noisy environment (the subjects listened to an audiotape recording of CCU sounds): treatment group …
References
- Campbell DT, Stanley J. Experimental and Quasi-Experimental Designs for Research. 1st Edition. Cengage Learning; 1963.
- Shadish WR, Cook TD, Campbell DT. Experimental and Quasi-Experimental Designs for Generalized Causal Inference. 2nd Edition. Cengage Learning; 2001.
Further Reading
One-Group Posttest only Design
- In a one-group posttest only design, a treatment is implemented (or an independent variable is manipulated) and then a dependent variable is measured once after the treatment is implemented. Imagine, for example, a researcher who is interested in the effectiveness of an anti-drug education program on elementary school students’ attitudes toward ill...
One-Group Pretest-Posttest Design
- In a one-group pretest-posttest design, the dependent variable is measured once before the treatment is implemented and once after it is implemented. Let’s return to the example of a researcher who is interested in the effectiveness of an anti-drug education program on elementary school students’ attitudes toward illegal drugs. The researcher could measure the at…
Interrupted Time Series Design
- A variant of the pretest-posttest design is the interrupted time-series design. A time series is a set of measurements taken at intervals over a period of time. For example, a manufacturing company might measure its workers’ productivity each week for a year. In an interrupted time series-design, a time series like this one is “interrupted” by a treatment. In one classic example, the treatment …