statisticalrethinkin..

310 12. MONSTERS AND MIXTURESWhy did I subtract the new additive portion from the intercept? If you look back atthe code for dordlogit, you’ll see that I’ve already snuck this assumption past you once.e reason for doing this is to make the parameter estimates easier to interpret, later. Wewould like a positive estimate for, say, β A to indicate an increase in the average value of y i ,when A i increases. But in order to increase the average y i , we need to bunch up probabilityon the high end of the distribution. is corresponds to reducing the cumulative log-oddsof every possible outcome value, leaving the remaining probability mass at the high end. Iknow this is weird. It will make sense once we estimate this model and plot its predictions,across changes in the predictor variables. So bear with me.You fit this model just as you’d expect, by adding the slopes and predictor variables tothe phi parameter inside dordlogit. Here’s a working model:R code12.7m11.2