[R] Baffled by drop1

From: George W. Gilchrist <gwgilc_at_wm.edu>
Date: Fri 04 Mar 2005 - 01:46:17 EST


I've been experimenting with drop1 for my biostatistics class, to obtain the so-called Type III sums of squares. I am fully aware of the deficiencies of this method, however I feel that the students should be familiar with it. What I find baffling is that when applied to a fully balanced design, you obtain different sums of squares. I've used this for several years in Splus and R and never encountered this before. Am I off my rocker? I have read the help files and remain clueless. Thanks for any advice.

> replications(Wt~Sex*Par.trt, data=tmp1)
$Sex
[1] 22

$Par.trt
[1] 22

$"Sex:Par.trt"

   Par.trt
Sex N S
  F 11 11
  M 11 11

> summary(tmp2<-aov(Wt~Sex*Par.trt, data=tmp1))

            Df Sum Sq Mean Sq F value    Pr(>F)
Sex          1 215600  215600 31.5367 1.638e-06 ***
Par.trt      1   7127    7127  1.0425    0.3134
Sex:Par.trt 1 19236 19236 2.8138 0.1013 Residuals 40 273459 6836
---
Signif. codes:  0 `***' 0.001 `**' 0.01 `*' 0.05 `.' 0.1 ` ' 1

> drop1(tmp2, ~., test="F")
Single term deletions Model: Wt ~ Sex * Par.trt Df Sum of Sq RSS AIC F value Pr(F) <none> 273459 392 Sex 1 53018 326477 398 7.7552 0.008144 ** Par.trt 1 1473 274932 391 0.2154 0.645067 Sex:Par.trt 1 19236 292695 393 2.8138 0.101256 --- Signif. codes: 0 `***' 0.001 `**' 0.01 `*' 0.05 `.' 0.1 ` ' 1
>
================================================================== George W. Gilchrist Email #1: gwgilc@wm.edu Department of Biology, Box 8795 Email #2: kitesci@cox.net College of William & Mary Phone: (757) 221-7751 Williamsburg, VA 23187-8795 Fax: (757) 221-6483 http://gwgilc.people.wm.edu/ ______________________________________________ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
Received on Fri Mar 04 09:34:03 2005

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