Characteristics of Experimental Design

In a previous post, we began a discussion on experimental design. In this post, we will begin a discussion on the characteristics of experimental design. In particular, we will look at the following

  • Random assignment
  • Control over extraneous variables

Random Assignment

After developing an appropriate sampling method, a researcher needs to randomly assign individuals to the different groups of the study. One of the main reasons for doing this is to remove the bias of individual differences in all groups of the study.

For example, if you are doing a study on intelligence. You want to make sure that all groups have the same characteristics of intelligence. This helps for the groups to equate or to be the same. This prevents people from saying the reason there are differences between groups is because the groups are different and not because of the treatment.

Control Over Extraneous Variables

Random assignment directly leads to the concern of controlling extraneous variables. Extraneous variables are any factors that might influence the cause and effect relationship that you are trying to establish. These other factors confound or confuse the results of a study. There are several methods for dealing with this as shown below

  • Pretest-posttest
  • Homogeneous sampling
  • Covriate
  • Matching


A pre-test post-test allows a researcher to compare the measurement of something before the treatment and after the treatment. The assumption is that any difference in the scores of before and after is due to the treatment.Doing the tests takes into account the confounding of the different contexts of the setting and individual characteristics.

Homogeneous Sampling

This approach involves selecting people who are highly similar on the particular trait that is being measured. This removes the problem of individual differences when attempting to interpret the results. The more similar the subjects in the sample are the more controlled the traits of the people are controlled for.


Covariates is a statistical approach in which controls are placed on the dependent variable through statistical analysis. The influence of other variables are removed from the explained variance of the dependent variable. Covariates help to explain more about the relationship between the independent and dependent variables.

This is a difficult concept to understand. However, the point is that you use covariates to explain in greater detail the relationship between the independent and dependent variable by removing other variables that might explain the relationship.


Matching is deliberate, rather than randomly, assigning subject to various groups. For example, if you are looking at intelligence. You might match high achievers in both groups of the study. By placing he achievers in both groups you cancel out there difference.


Experimental design involves the cooperation in random assignment of inclusive differences in a sample. The goal of experimental design is to be sure that the sample groups are mostly the same in a study. This allows for concluding that what happened was due to the treatment.

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