Hi Christian,
please give always reproducible code,
so we can see what have done
and give you the best answer.
lm function, generally
as in this example form lm man page ( ?lm)
> trt <- c(4.81,4.17,4.41,3.59,5.87,3.83,6.03,4.89,4.32,4.69)
>ctl <- c(4.17,5.58,5.18,6.11,4.50,4.61,5.17,4.53,5.33,5.14)
>reg=lm(trt~ctl)
>summary(reg)
Call:
lm(formula = trt ~ ctl)
Residuals:
Min 1Q Median 3Q Max
-1.09389 -0.33069 -0.15249 0.05128 1.45497
Coefficients:
Estimate Std. Error t value Pr(>|t|)
(Intercept) 7.7957 2.1661 3.599 0.00699 **
ctl -0.6230 0.4279 -1.456 0.18351
---
Signif. codes: 0 â***â 0.001 â**â 0.01 â*â 0.05 â.â 0.1 â
â 1
Residual standard error: 0.7485 on 8 degrees of freedom
Multiple R-squared: 0.2095, Adjusted R-squared: 0.1106
F-statistic: 2.12 on 1 and 8 DF, p-value: 0.1835
Returns you all the answer (almost) for the questions that you ask;
the p-value of the intercept line, is the p-value from the
test( t test) if the intercept is different form zero.
the ctl line has also the same interpretation, regarding the value returned.
Meaning no is not significantly different form zero.
If you want to test if the estimates ( slopes or intercept) are
different from a specific value as in your case different for 0.5
you can apply a test.
Type on R
?t.test
and you can find the all the information you need.
Hope this helps
Best Regards
Anna
Anna Freni Sterrantino
Ph.D Student
Department of Statistics
University of Bologna, Italy
via Belle Arti 41, 40124 BO.
________________________________
Da: Christian Arnold <[EMAIL PROTECTED]>
A: r-help@r-project.org
Inviato: Martedì 9 dicembre 2008, 21:54:23
Oggetto: [R] Significance of slopes
Hello R community,
I have a question regarding correlation and regression analysis. I have two
variables, x and y. Both have a standard deviation of 1; thus, correlation and
slope from the linear regression (which also must have an intercept of zero)
are equal.
I want to probe two particular questions:
1) Is the slope significantly different from zero? This should be easy with the
lm function, as the p-value should reflect exactly that question. If I am
wrong, lease correct me.
2) Is the slope significantly different from a non-zero value (e.g. 0.5)? How
can I probe that hypothesis? Any ideas?
I apologize if this question is too trivial and already answered somewhere, but
I did not find it.
[[elided Yahoo spam]]
Christian
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