Re: [R] Type II and III sum of square in Anova (R, car package)

2006-08-29 Thread Bill.Venables
I cannot resist a very brief entry into this old and seemingly immortal issue, but I will be very brief, I promise! Amasco Miralisus suggests: As I understood form R FAQ, there is disagreement among Statisticians which SS to use

Re: [R] Type II and III sum of square in Anova (R, car package)

2006-08-29 Thread John Sorkin
Amasco, In general it is dangerous to attempt to interpret a main effect that is included in an interaction, regardless of wether or not the interaction is significant. If you want to make a valid inference about a main effect it is safest to do so after dropping any interaction that contains the

Re: [R] Type II and III sum of square in Anova (R, car package)

2006-08-28 Thread Amasco Miralisus
Hello, First of all, I would like to thank everybody who answered my question. Every post has added something to my knowledge of the topic. I now know why Type III SS are so questionable. As I understood form R FAQ, there is disagreement among Statisticians which SS to use

Re: [R] Type II and III sum of square in Anova (R, car package)

2006-08-28 Thread John Fox
Dear Amasco, Again, I'll answer briefly (since the written source that I previously mentioned has an extensive discussion): -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Amasco Miralisus Sent: Monday, August 28, 2006 2:21 PM To:

Re: [R] Type II and III sum of square in Anova (R, car package)

2006-08-27 Thread Prof Brian Ripley
I think this starts from the position of a batch-oriented package. In R you can refit models with update(), add1() and drop1(), and experienced S/R users almost never use ANOVA tables for unbalanced designs. Rather than fit a pre-specified set of sub-models, why not fit those sub-models that

Re: [R] Type II and III sum of square in Anova (R, car package)

2006-08-27 Thread John Fox
Dear Amasco, A complete explanation of the issues that you raise is awkward in an email, so I'll address your questions briefly. Section 8.2 of my text, Applied Regression Analysis, Linear Models, and Related Methods (Sage, 1997) has a detailed discussion. (1) In balanced designs, so-called Type

Re: [R] Type II and III sum of square in Anova (R, car package)

2006-08-26 Thread Mark Lyman
1. First of all, more general question. Standard anova() function for lm() or aov() models in R implements Type I sum of squares (sequential), which is not well suited for unbalanced ANOVA. Therefore it is better to use Anova() function from car package, which was programmed by John Fox to use