On 22.05.2013 07:09, meng wrote:
Thanks.
As to the data " warpbreaks", if I want to analysis the impact of
tension(L,M,H) on breaks, should I order the tension or not?
No homework questions on this list, please ask your teacher.
Best,
Uwe Ligges
Many thanks.
At 2013-05-21 20:55:18,"David Winsemius" <dwinsem...@comcast.net> wrote:
On May 20, 2013, at 10:35 PM, meng wrote:
Hi all:
If the explainary variables are ordinal,the result of regression is different
from
"unordered variables".But I can't understand the result of regression from
"ordered
variable".
The data is warpbreaks,which belongs to R.
If I use the "unordered variable"(tension):Levels: L M H
The result is easy to understand:
Estimate Std. Error t value Pr(>|t|)
(Intercept) 36.39 2.80 12.995 < 2e-16 ***
tensionM -10.00 3.96 -2.525 0.014717 *
tensionH -14.72 3.96 -3.718 0.000501 ***
If I use the "ordered variable"(tension):Levels: L < M < H
I don't know how to explain the result:
Estimate Std. Error t value Pr(>|t|)
(Intercept) 28.148 1.617 17.410 < 2e-16 ***
tension.L -10.410 2.800 -3.718 0.000501 ***
tension.Q 2.155 2.800 0.769 0.445182
What's "tension.L" and "tension.Q" stands for?And how to explain the result
then?
Ordered factors are handled by the R regression mechanism with orthogonal polynomial contrasts:
".L" for linear and ".Q" for quadratic. If the term had 4 levels there would also have
been a ".C" (cubic) term. Treatment contrasts are used for unordered factors. Generally one would
want to do predictions for explanations of the results. Trying to explain the individual coefficient values
from polynomial contrasts is similar to and just as unproductive as trying to explain the individual
coefficients involving interaction terms.
--
David Winsemius
Alameda, CA, USA
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