# [R] Baffled by drop1: Please ignore previous request!

From: George W. Gilchrist <gwgilc_at_wm.edu>
Date: Fri 04 Mar 2005 - 02:07:53 EST

My apologies to the list for sending this without adequate research. I have found my answer; please ignore! Thanks.

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.

\$Par.trt
[1] 22

\$"Sex:Par.trt"

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

```            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/

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