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Mixture Regression Modeling
It would be a mistake to try to fit a single regression line to the whole sample. On the
other hand, two straight lines, one for each group, would fit the data well. This is a job
for mixture regression modeling. A mixture regression analysis would attempt to
divide the sample up into groups and to fit a separate regression line to each group.
Second Dataset
The following dataset, in the file DosageAndPerformance2.sav, also contains data on
the variables dosage, performance, and group.
Again, a scatterplot of the data shows evidence of two groups, with each group
requiring its own regression line. In either group by itself, an increase of one unit in
dosage is associated with an increase of about two units in performance, so that the
slope of the regression line is about 2 within each group. On the other hand, the two
groups have different intercepts. At any particular dosage level, performance is 5
points or so higher in one group than in the other. A mixture regression analysis of this
dataset would attempt to divide the sample up into groups and to fit a separate
regression line to each group.