The between-subject or independent-measures experimental design is a kind of strategy that allows the researcher to obtain two or more different scores from different groups of participants through the manipulation of the independent variable and then comparing the score results. This strategy is used in experimental research permitting to achieve different results by two or more different treatment conditions in the same experiment, which every participant is only subjected to a single treatment.
This strategy has a significant advantage as decreasing the chances of participants suffering boredom after a long series of tests or becoming more accomplished through practice and experience, creating biased results. However, even though it is widely used in psychology's experiment, this strategy presents disadvantages due to the presence of confounding variables.
Individual differences (assignment bias) and Environmental variables primarily represent this negative issue. Individual differences pose a significant challenge because it holds all the perceived fundamental differences that characterize each participant, like gender, race, social background, economic status, intelligence level, etc.). All these variables can become a confounding variable that can represent a critical threat to internal validity. Confounding variable is likely to occur when a personal characteristic is distributed unequally among the groups being compared. The individual differences may confer differences between scores of two groups that are not caused by the different treatment conditions, but for the simply extraneous variables that are not adequately addressed in the research design. Moreover, individual differences can promote a high variability within the treatment compromising the research results. Equivalent groups of participants protect the study from assignment bias, and it is valid by the use of a few techniques:
-Randomization- Although it can be a problem in small samples, this method ensures that the distribution of participant is equally making it possible through restrict random assignment. Restriction limits generalizability and allows that groups have the same number of participants as well.
- Matching Groups- Ensures that the different group participants are equivalent or nearly equivalent in the same proportion that exists in the sample. The procedure of match process depends on the identification the variable to be matched, measure it for each participant and then assigning participants groups in a way that guarantee the balance between groups. It is important to acknowledge that it can be time-consuming and expensive.
- Holding Variables Constant-It guarantees the restrict of the range of the variable, what represents a variable to become a confounding factor in the research study. For example, in the hypothetical study looking at the association between frustration and aggressiveness, suppose that age and gender were the only two confounders of concern. If so, confounding by these factors could have been avoided by making sure that all subjects were males between the ages of 18-25. It will ensure that the age distributions are similar in the groups being compared, so that confounding will be minimized. The drawback of this method is that it compromises the ability to generalize results to the large population, an issue that represents as a threat to external validity.
Finally, to avoid environment bias, it is necessary to account the interference of the place and time on groups of participants that can lead to an inaccurate measurement obscured by a poor design which disregards that people responding differently according to periods of the day like during early morning and lunchtime, for example.
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