Experimental Design: Within Groups

Within groups experimental designs is the use of only one group in an experiment. This is in contrast to a between group design which involves two or more groups. Within group design is useful when the number of is two low in order to split them into different groups.

There are two common forms of within group experimental design, time series and repeated measures. Under time series there are interrupted times series and equivalent time series. Under repeated measure there is only repeated measure design. In this post we will look at the following forms of within group experimental design.

  • interrupted time series
  • equivalent time series
  • repeated measures design

Interrupted Time Series Design

Interrupted time series design involves several pre-test followed by an intervention and then several post-test of one group. By measuring the several times, many threats to internal validity are reduced, such as regression, maturation, and selection. The pre-test results are also used as covariates when analyzing the post-tests.

Equivalent Time Series Design

Equivalent time series design involves the use of a measurement followed by intervention followed by measurement etc. In many ways, this design is a repeated post-test only design. The primary goal is to plot the results of the post-test and determine if there is a pattern that develops over time.

For example, if you are tracking the influence of blog writing on vocabulary acquisition, the intervention is blog writing and the dependent variable is vocabulary acquisition. As the students write a blog, you measure them several times over a certain period. If a plot indicates an upward trend you could infer that blog writing made a difference in vocabulary acquisition.

Repeated Measures

Repeated measures is the use of several different treatments over time. Before each treatment, the group is measured. Each post-test is compared to other post-test to determine which treatment was the best.

For example, lets say that you still want to assess vocabulary acquisition but want to see how blog writing and public speaking affect it. First, you measure vocabulary acquisition. Next, you employ the first intervention followed by a second assessment of vocabulary acquisition. Third, you use the public speaking intervention followed by a third assessment of vocabulary acquisition. You now have three parts of data to compare

  1. The first assessment of vocabulary acquisition (a pre-test)
  2. The second assessment of vocabulary acquisition (post-test 1 after the blog writing)
  3. The third assessment of vocabulary acquisition (post-test 2 after the public speaking)


Within group experimental designs are used when it is not possible to have several groups in an experiment. The benefits include needing fewer participants. However, one problem with this approach is the need to measure several times which can be labor intensive.


One thought on “Experimental Design: Within Groups

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