R help - Mar 2008 - (no subject)

# PROBLEM WITH ABLINE

# I have a question about a seemingly imprecise abline figure which I have
come accross now a few times.
# After reading variables a and b...
a<-c(3729,1796,1956,5177,3201,3138,4265,3042,3745,2060,5713,6811,4184,1221,4700,3171,8913,5565,5500,2428,4841,10428,11197,5946,2783,4842)
b<-c(7,3.4,4.8,11.2,7,5,8.2,6.2,7.4,7.4,18.4,13.8,8.8,3.6,7.2,8.8,23.2,11.8,10.4,5.4,13.7,26.4,27.8,12.8,6.2,15.4)
# ... I do a simple linear regression...
model<-lm(a~b)
summary(model)
# and plot it.
plot(a~b)
abline(model)
# abline drawn with the parameters intercept and slope taken from
summary(model) yield exactly the same.
abline(767.59,356.9)
predict(model,list(b=10))
# At b=10, the predicted value of a is 4336.543.
# Now my puzzling question comes: if I deduce the value of variable a from
b=10 from the already created figure,
# I conclude that a is about 4000 and clearly not something that even comes
close to 4300.

# If I plot only a small part of the regression, the line looks much better
although still there is a gap
# between what I observe in the figure and what is the calculated
prediction.
plot(a~b,xlim=c(10,12),ylim=c(4000,4500))
abline(model)
# If I do the prediction by hand
# from the regression results (summary(model)) then the same figure as the
calculated prediction is the result.
# My preliminar conclusion is that the figure is imprecise and that the
calculations are correct.

# So my QUESTION is: is abline imprecise or am I making errors?


	[[alternative HTML version deleted]]

imprecise are our eyes :)

check where 4336.543 actually is and note how the range of the data  
confused you.

plot(a~b)
abline(model)
abline(h=4336.543, v=10)

b

On Mar 11, 2008, at 11:27 AM, Aad Termorshuizen wrote:
>
> # PROBLEM WITH ABLINE
>
> # I have a question about a seemingly imprecise abline figure which  
> I have
> come accross now a few times.
> # After reading variables a and b...
> a<- 
> c 
>
(3729,1796,1956,5177,3201,3138,4265,3042,3745,2060,5713,6811,4184,1221,4700,3171,8913,5565,5500,2428,4841,10428,11197,5946,2783,4842
> )
> b<- 
> c 
>
(7,3.4,4.8,11.2,7,5,8.2,6.2,7.4,7.4,18.4,13.8,8.8,3.6,7.2,8.8,23.2,11.8,10.4,5.4,13.7,26.4,27.8,12.8,6.2,15.4
> )
> # ... I do a simple linear regression...
> model<-lm(a~b)
> summary(model)
> # and plot it.
> plot(a~b)
> abline(model)
> # abline drawn with the parameters intercept and slope taken from
> summary(model) yield exactly the same.
> abline(767.59,356.9)
> predict(model,list(b=10))
> # At b=10, the predicted value of a is 4336.543.
> # Now my puzzling question comes: if I deduce the value of variable  
> a from
> b=10 from the already created figure,
> # I conclude that a is about 4000 and clearly not something that  
> even comes
> close to 4300.
>
> # If I plot only a small part of the regression, the line looks much  
> better
> although still there is a gap
> # between what I observe in the figure and what is the calculated
> prediction.
> plot(a~b,xlim=c(10,12),ylim=c(4000,4500))
> abline(model)
> # If I do the prediction by hand
> # from the regression results (summary(model)) then the same figure  
> as the
> calculated prediction is the result.
> # My preliminar conclusion is that the figure is imprecise and that  
> the
> calculations are correct.
>
> # So my QUESTION is: is abline imprecise or am I making errors?
>
>
> 	[[alternative HTML version deleted]]
>
> ______________________________________________
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> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.

On Tuesday 11 March 2008 04:27:12 pm Aad Termorshuizen wrote:
AT> # So my QUESTION is: is abline imprecise or am I making errors?

Either you are not precise or I did not got your problem.

plot(a~b)
abline(model)
points(10,4336,col="red")

as you see the predicted point (red) is exactly at the abline-line.

Stefan
-- 
Microeconomics
University of Erfurt