In one study (Hedrick et al., 1993), researchers compared the number of motorcycle fatalities that occurred before helmets were mandatory, after helmets were mandatory, and after the helmet laws were repealed. This study used an format with the interrupted time-series design.

Independent groups design format

Explanation:

In this example there is only a single variable and it is the number of persons that were not selected specifically for the study. These groups are not equivalent in age, sex, etc and they are from different residential areas and of varying health and ethnic background.

The essence of these tests is to design a single hypothesis over a single variable. In this case wearing or not wearing a motorcycle helmet. It has been found in multiple places that the incidences of road traffic accident deaths reduced with wearing helmets.

Since there was a strong correlation between these two data sets it can be said that they are influenced by each other.