Regression Discontinuity Design

Open the file rd.sav.  This file contains data on 39 children who were assigned to experimental and control groups as part of the evaluation. The program was designed to reduce cognitive deficiency by providing nutrition supplements.  The funder required that the experimental group be comprised of those who were most “needy.”  Therefore, children whose score on a height-weight index fell below 70 were assigned to the experimental group, and those whose score was 70 or above were assigned to the control group.

1. Regress Posttest on Program and Assign to assess the impact of the program. 

a)     State the equation representing the line that best fits these data   (Y = …).

b)     Based on these results, what is the impact of the program on cognitive deficiency?

2. Regress Posttest on Program for each of the groups separately (program and control). 

a)     Are the slopes the same for the two groups?

b)     Draw a graph that shows what you found.

c)     If the slopes are not the same, what might this suggest about the true relationship between the Assign and Outcome variables?  What are the implications for the evaluation?