# [Rd] under certain conditions, model.matrix appears to lack one column (PR#646)

**rnassar@duke.edu
**
rnassar@duke.edu

*Sun, 27 Aug 2000 22:24:00 +0200 (MET DST)*

Dear R Team,
# Summary of the problem: setting contrasts as
> contrasts(g) <- contr.treatment
or > contrasts(g) <- matrix(c(1,-1,0),ncol=1)
(i.e. without quotes around `contr.treatment' or `contr.sum', etc.)
and fitting an lm model without an intercept results in a model matrix
that lacks one column.
(I do ask for forgiveness if this is not a bug but is due to my
ignorance.)
Many thanks for the extraordinary R.
Rashid Nassar
# Details:
# the data:
x <- gl(3,3)
y <- c(2,4,3,4,5,7,6,5,8)
## 1. Using the default (treatment) contrasts unchanged gives correct
results.
## 2. Setting contrasts using either of the following two forms also
gives correct results:
>* contrasts(x) <- "contr.treatment"
*>* contrasts(C(x,contr.treatment))
*
## 3. However I believe that the following (contr.treatment without quotation
marks) results in a problem (also contrasts(x) <- matrix(c(1,-1,0),ncol=1))
when an intercept is not included in the model:
>* contrasts(x) <- contr.treatment
*>* f4 <- lm(y ~ x - 1)
*>* summary(f4)
*
Coefficients:
Estimate Std. Error t value Pr(>|t|)
x2 5.333 1.351 3.947 0.00555
x3 6.333 1.351 4.688 0.00224
>* anova(f4)
*Analysis of Variance Table
Response: y
Df Sum Sq Mean Sq F value Pr(>F)
x 2 205.667 102.833 18.778 0.001537
Residuals 7 38.333 5.476
>* model.matrix(f4)
* x2 x3
1 0 0
2 0 0
3 0 0
4 1 0
5 1 0
6 1 0
7 0 1
8 0 1
9 0 1
--please do not edit the information below--
Version:
platform = i586-pc-linux-gnu
arch = i586
os = linux-gnu
system = i586, linux-gnu
status =
major = 1
minor = 1.1
year = 2000
month = August
day = 15
language = R
Search Path:
.GlobalEnv, package:MASS, Autoloads, package:base
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