Nominal/Ordinal logistic regression

[#1]

I’m using a logistic regression nominal model for a response variable (pregnant/not-pregnant). My independent variables are: Time of insemination + Production type + interaction. When TOI (48, 52, 56h) is described as a nominal variable both PT and the interaction are significant. When TOI is used is described as a ordinal variable, only the interaction is significant. Why is this relevant to the analysis? I thought the type of logistic regression to be used (nominal or ordinal) was determined only by the data nature of the response variable. Many thanks for any help I can get.

[#2 · Accepted solution]

To quote from the 'Statistical Details' section in the 'Fitting Linear Models' document "Factors marked with the ordinal modeling type are coded differently than nominal factors. The parame...​".

Take a look at:

http://www.jmp.com/support/help/The_Factor_Models.shtml#93454

for all the details.

[#3]

Ian, It took me this long to thank you... sorry. Much appreciated your comments on my question. --Erick