How we change what others think, feel, believe and do
The regression-discontinuity design uses a cut-off selection method, for example where subjects are selected based on scoring above or below a certain value on a previous test.
In design notation, this is:
The key aspect about this design is that the control group is made up of those who fall at the other side of the cut-off score. The sample is thus cut in two, with one group as control and the rest as the treatment group.
A training module is designed to increase the visual-spatial ability of people with lower 'IQ'. A sample is selected and tested for IQ, with those scoring below 100 being allocated to the cut-off group.
A test for visual-spatial ability was then administered before the lower-IQ group was given the training. A post-test score showed that their ability had increased in this area.
This is a relatively unusual design that is very useful in specific situations.
In practice, if the cut-off selects only a small number, then the control group may also be constrained. In the same way, more subjects may be sought for the control group if the treatment group takes a significant majority.
The name 'regression discontinuity' arises because the assignment by score causes a discontinuity across this score boundary between pre-test and post-test and between treatment and control groups, as in the diagram below.
And the big