[gov2001-l] Re: ordered probit coefficients

Excellent questions, mike. 1) Just so everyone is clear on this: polr() is for ordinal *logit* not ordinal probit. 2) The difference is one in parameterization. Remember how we fixed tau1 = 0? If you fix tau1 to a different value, you will get a different answer for the second intercept term (tau2) and different signs on the coefficients (beta). As long as you use the same parameterization to calculate the quantities of interest, the predicted probability of falling into each bin will be invariant to reparameterization of the systematic component. 3) The substantive question of interest can't be the unobserved underlying variable (because the stochastic component is multinomial). Thus, the expected value for this model is the predicted probability that an observation i falls into each bin. So let's say that you're interested in presidential approval. Someone conducts a survey asking: "The president is doing an effective job. Strongly agree (1), agree (2), neutral (3), disagree (4), strongly disagree (5)." We observe the number of people in bins 1:5 and estimate the model and find beta and tau2-tau5. If we were just interested in the unobserved underlying distribution Y*, the quantity we would calculate is x'beta = mu. Now what does mu mean? mu relative to what? It has to be mu relative to the cut points. Olivia ----- Original Message ----- From: "Michael Richard Kellermann" <kellerm(a)fas.harvard.edu> To: <gov2001-l(a)lists.fas.harvard.edu> Sent: Thursday, December 16, 2004 10:24 AM Subject: [gov2001-l] Re: ordered probit coefficients