What is the difference between the classic experimental design and the posttest only control group design?
Pretest–Posttest Design Show
The basic premise behind the pretest–posttest design involves obtaining a pretest measure of the outcome of interest prior to administering some treatment, followed by a posttest on the same measure after treatment occurs. Pretest–posttest designs are employed in both experimental and quasi-experimental research and can be used with or without control groups. For example, quasi-experimental pretest–posttest designs may or may not include control groups, whereas experimental pretest–posttest designs must include control groups. Furthermore, despite the versatility of the pretest–posttest designs, in general, they still have limitations, including threats to internal validity. Although such threats are of particular concern for quasi-experimental pretest–posttest designs, experimental pretest–posttest designs also contain threats to internal validity. Types of Pretest–Posttest Designs without Control GroupsOne-Group Pretest-Posttest DesignIn the simplest pretest–posttest design, researchers gather ... locked icon Sign in to access this contentSign in Get a 30 day FREE TRIAL
sign up today! 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. The intervention can be:
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. Advantages of the posttest-only control group design1. The treatment and control groups are equivalent at baselineThis 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 be. As long as we are assigning participants at random, we won’t be subject to selection bias and we will obtain comparable groups. 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
3. Can be used when participants’ anonymity must be keptWhen subjects are measured before and after the intervention, some sort of system should be installed in order to know which measurement corresponds to which participant. In these designs, the participant’s ID, name or phone number will be recorded in a database which may interfere with some type of studies where the participant’s anonymity is critical. 4. Not affected by reactions to pretestingAn additional source of bias may be present when posttest results can be influenced by the results of a pretest. For instance:
So a study involving a pretest may be measuring the effect of the intervention along with this test-retest effect (a.k.a. sensitization to pretest). As this design does not have any pretest, it will not be subject to this bias. 5. Can be done when a pretest is not possibleThis is especially useful when the measurement itself is very expensive either in terms of time or money, or when its results can be easily predicted or constant. 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. Limitations of the posttest-only control group design1. High risk of attrition biasThis 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 absence of a pretest makes it very hard to detect and control this bias. 2. The effect of the intervention on subgroups is not clearIn 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. 3. Requires a large sample sizeThe minimum sample size required for this study will be larger compared to studies without random assignment (i.e. quasi-experiments), this is because randomness can correct the differences between the treatment and control groups better as the sample gets larger. 4. Less generalizable than observational designsLow external validity is a general characteristic of experiments for 2 reasons:
Observational studies however, do not have such limitations as the investigator is just an observer of natural events, watching and recording them as they happen without controlling or influencing them. In general, a highly controlled study will have a high internal validity (i.e. less bias) and a low external validity (i.e. low generalizability). Example of a study that used a posttest-only control group designIn 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. So they randomly assigned 70 women with no hearing or sleeping problems to attempt to sleep in one of the following conditions:
Note that this experiment was done in a sleep laboratory. Their results showed that CCU sounds can cause poorer REM sleep. What can we learn from this example?
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Further reading
What is the difference between the two types of experimental design?1. In experimental research, the researcher can control and manipulate the environment of the research, including the predictor variable which can be changed. On the other hand, non-experimental research cannot be controlled or manipulated by the researcher at will.
What is the posttestThe posttest-only control group design is a research design in which there are at least two groups, one of which does not receive a treatment or intervention, and data are collected on the outcome measure after the treatment or intervention.
What is the difference between the control group and the experimental?What is the difference between a control group and an experimental group? An experimental group, also known as a treatment group, receives the treatment whose effect researchers wish to study, whereas a control group does not. They should be identical in all other ways.
What is the difference between a posttestA posttest is an assessment measure given to participants after they have received treatment as part of a research study. A pretest-posttest research design must provide participants with the same assessment measures before and after treatment in order to determine if any changes can be connected to the treatment.
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