This add-in ingests estimates from a Fit Model report of a one-way ANCOVA with interaction to generate a combined data table of indicator parameterization estimates treating each level of the categorical variable as the reference category successively. This allows for testing of pairwise slopes in this analysis.
Instructions (after installing add-in):
Expanded details and example:
An ANCOVA including an interaction is a type of multiple regression in which one continuous Y is modeled by one categorical X, one continuous X, and the interaction between the two. A traditional ANCOVA does not include this interaction (and a critical assumption is the homogeneity of regression) but sometimes it is the case that there is evidence the slopes between Y and the continuous X differ across the levels of the categorical X (e.g. a statistically significant interaction or prior theory predict this). When there are 3 or more levels of the categorical predictor, it's useful to follow-up on this interaction with pairwise tests of slopes to see for which groups the slopes differ. In the example below I'll use the US Demographics sample data table available in the sample data directory.
First, let's fit a model using Analyze > Fit Model. Here I will predict Eighth Grade Math (continuous) on the basis of Region (nominal), High School Graduates (continuous), and the interaction of the two. This is the only structure this add-in currently supports (i.e. continuous ~ nominal + continuous + nominal*continuous)
In the regression plot we see regression lines for Eighth Grade Math against High School Graduates for each region.
Several of these regions appear to have a similar regression, but the slope of the south appears to be a bit different. Our overall effect test for Region*High School Graduates does not give us substantial evidence there is an interaction (p = 0.3302) so we might simply regard this difference in slope for the south to be random error. For this example, let's follow-up on these slopes anyway and produce pairwise tests of the slopes. To do this, run the add-in, Add-ins > One Way ANCOVA with Interaction Simple Slopes Test. This will produce the table of estimates below:
This table was constructed by requesting indicator parameter estimates for this model (estimates that reflect deviations from a reference category) treating each region as the reference. The Reference Category column indicates which level of Region is used as the reference for each row.
There are four types of estimates in this table:
Revision History:
v0.02 - Added support dialog to select from multiple least squares reports
v0.03, v0.04 - Fixed defect: script failed when nominal column was numeric
v0.05 - Fixed defect: when partial value ordering was set additional levels were ignored.
Hi,
I tried running this add in and it didn't give any new table of results.
Looking at the Log i found that an error is in the line
reDoScript = Get Clipboard() /*###*/;
what could be the problem?
Thanks, Eli
Hello,
Very useful addin, thank you very much!
I have a question concerning the p-values. I know that for some tests performed in JMP, the p-values are corrected for multiple comparaisons. Is this the case here?
Also, is the test still valid for a situation in which the categorical variable would be Y and not X (opposite to what's explained in the expanded details)?
Thank you,
Andreea