Dear Balavelan, First I would suggest to include the main effects of all variables in an interaction. So you model with interaction should be y ~ a + b + c + d + c:d. Which can be abbreviated to y ~ a + b + c*d.
Futhermore you should take a look at ?predict.glm and pay attention to the newdata and type = "reponse" arguments. HTH, Thierry ------------------------------------------------------------------------ ---- ir. Thierry Onkelinx Instituut voor natuur- en bosonderzoek / Research Institute for Nature and Forest Cel biometrie, methodologie en kwaliteitszorg / Section biometrics, methodology and quality assurance Gaverstraat 4 9500 Geraardsbergen Belgium tel. + 32 54/436 185 [EMAIL PROTECTED] www.inbo.be To call in the statistician after the experiment is done may be no more than asking him to perform a post-mortem examination: he may be able to say what the experiment died of. ~ Sir Ronald Aylmer Fisher The plural of anecdote is not data. ~ Roger Brinner The combination of some data and an aching desire for an answer does not ensure that a reasonable answer can be extracted from a given body of data. ~ John Tukey -----Oorspronkelijk bericht----- Van: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Namens Bala Thanigaivelan Verzonden: donderdag 3 april 2008 4:22 Aan: r-help@r-project.org Onderwerp: [R] need help with understanding stepAIC Hi, I am currently using R extensively to develop a statistical model and currently learning its capabilities. I am trying to develop a logistic regression model using glm and stepAIC I constructed my formula as y ~ a + b + c + d Now I wish to study the interaction between variables. So I did y ~ a + b + c:d + d The software generated the coefficients for a, b , c:d, d Now I want to compute the probability for various events using the logistic regression model formula Prob = exp(y)/(1+exp(y)) How do I compute the value of y for the formula with interaction variables ? what does c:d represent given a dataset ? Regards, Balavelan [[alternative HTML version deleted]] ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code. ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
