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.
Random Assignment
- 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
Random Sampling
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.
- Module 2:
- Objectives
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- Section 1
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- Section 1 Discussion
- Section 2
- Section 2 Discussion
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- Quiz
Section 2: Experimental Studies
Unlike a descriptive study, an experiment is a study in which a treatment, procedure, or program is intentionally introduced and a result or outcome is observed. The American Heritage Dictionary of the English Language defines an experiment as "A test under controlled conditions that is made to demonstrate a known truth, to examine the validity of a hypothesis, or to determine the efficacy of something previously untried."
This means that no matter who the participant is, he/she has an equal chance of getting into all of the groups or treatments in an experiment. This process helps to ensure that the groups or treatments are similar at the beginning of the study so that there is more confidence that the manipulation [group or treatment] "caused" the outcome. More information about random assignment may be found in section Random assignment.
Definition: An experiment is a study in which a treatment, procedure, or program is intentionally introduced and a result or outcome is observed.
Case Example for Experimental Study
Experimental Studies — Example 1
An investigator wants to evaluate whether a new technique to teach math to elementary school students is more effective than the standard teaching method. Using an experimental design, the investigator divides the class randomly [by chance] into two groups and calls them "Group A" and "Group B." The students cannot choose their own group. The random assignment process results in two groups that should share equal characteristics at the beginning of the experiment.
Experimental Studies — Example 2
A fitness instructor wants to test the effectiveness of a performance-enhancing herbal supplement on students in her exercise class. To create experimental groups that are similar at the beginning of the study, the students are assigned into two groups at random [they can not choose which group they are in]. Students in both groups are given a pill to take every day, but they do not know whether the pill is a placebo [sugar pill] or the herbal supplement. The instructor gives Group A the herbal supplement and Group B receives the placebo [sugar pill]. The students' fitness level is compared before and after six weeks of consuming the supplement or the sugar pill. No differences in performance ability were found between the two groups suggesting that the herbal supplement was not effective.