On Sep 6, 2010, at 4:03 AM, B W wrote:
<Snipped out formatting detritus and added back many missing speces.>
->Hello,I am trying to take the information from the summary of my
best fit logisticregression model for the occurrence of a high
elevation plant spp. and create the appropriate equation that will
calculate probability of occurrence, given the data. My predictors
include both continuous variables (slope and a second
orderpolynomial of elevation) and a discrete variable for aspect
(warm and cool). I have left unchanged the default contrasts option,
so I believe that thefollowing coefficients were created using
treatment contrasts. My question how can I take this summary output
and create the logistic equation that will allow me to calculate
probability of occurrence. My interests are touse this to spatially
display this info in a GIS environment.
I think you should:
-- Read the Posting Guide where you should learn that this is a plain
text mailing list and that you need to change the configuration of
your mail client.
-- Read the help page and read other documentation regarding the use
of the predict function.
I have made adraft equation (shown below) that uses the coefficients
from this summaryoutput, but this appears to be incorrect values
always return zeroprobabilities. Presumably I need to adjust the
values in some way but I am unclear as to how to proceed.
Anyguidance would be appreciated!
>summary (
Call:glm(formula= Po ~ Slope + poly(Elevation, 2) + Aspect_2, family
= quasibinomial) DevianceResiduals: Min 1Q Median
3Q Max -1.0532 -0.4167 -0.2760 -0.1823 3.3376
Coefficients: Estimate Std. Error t valuePr(>|
t|) (Intercept) -4.577707 0.222406 -20.583 < 2e-16 ***
Slope 0.039959 0.003593 11.121 < 2e-16 ***
poly(Elevation,2)1 8.050898 5.601956 1.437 0.1508
poly(Elevation,2)2 -37.694521 6.297806 -5.985 2.39e-09 ***
Aspect_2w 0.429229 0.174760 2.456 0.0141 * ---
You may get predictions at the original data points with:
pred < predict(model.Slope.Elevation.Aspect)
(1/ (1 + exp(-1 * (-4.577707 + 0.039959*Slope + 8.050898 *
poly(Elevation, 2)1 + -37.694521 * poly(Elevation, 2)2 + 0.429229*
Aspect_2w)))))
Brendan Wilson
2530 Alexis Road
Shoreacres BC
Canada V1N 4P6
Ph: 1.250.359.5905
[[alternative HTML version deleted]]
David Winsemius, MD
West Hartford, CT
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