[R] Odp: Comparing 2 slopes of 2 regression lines

Petr PIKAL petr.pikal at precheza.cz
Mon Apr 6 12:56:21 CEST 2009


Hi

r-help-bounces at r-project.org napsal dne 06.04.2009 12:28:35:

> Hello everyone,
> I would like to test two regression slopes:do they differ 
significantly?The 
> data and commands I've used so far:
> x<-8.5:32.5   #Vektor x
> y<-c(   NA ,  NA , 5.67 , 6.53 , 6.83,  7.41 , 7.93 ,  8.5 , 8.86,  9.46 
, 9.
> 82 ,   10 ,10.35 , 10.7 ,11.03 ,11.37 ,11.61 ,11.84, 12.12, 12.39 ,12.67 
,12.
> 96, 13.28 ,13.47, 13.65) #Vektor y
> 
> (regression<-lm(y~x))
> summary(regression)   Regression model 1
> 
> x<-8.5:32.5   #Vektor x
> y<-c(  NA, 5.88  ,6.95 ,  7.2  ,7.66 , 8.02,  8.44,  9.06 , 9.65, 10.22 
,10.
> 63, 11.06, 11.37, 11.91, 12.28 ,12.69 ,13.07 , 13.5 , 13.3 ,14.14 ,   NA 
 , 
> NA  ,  NA  ,  NA   , NA) #Vektor y
> 
> (regression<-lm(y~x))
> summary(regression)     Regression model 2
> 
> Now I don't know how to continue as I want to test the slopes only ,not 
the 
> regression lines in total.

Here is one approach
http://tolstoy.newcastle.edu.au/R/e6/help/09/01/2632.html

> 
> Another problem I have is rather a formal one:
> y<-c(   NA ,  NA , 5.67 , 6.53 , 6.83,  7.41 , 7.93 ,  8.5 , 8.86,  9.46 
, 9.
> 82 ,   10 ,10.35 , 10.7 ,11.03 ,11.37 ,11.61 ,11.84, 12.12, 12.39 ,12.67 
,12.
> 96, 13.28 ,13.47, 13.65) #Vektor y
> 
> that is my y-vector; I always need to copy and paste this line 
originating
> from:
>       X8.5 X9.5 X10.5 X11.5 X12.5 X13.5 X14.5 X15.5 X16.5 X17.5
> SD24   NA   NA  5.67  6.53  6.83  7.41  7.93   8.5  8.86  9.46
> 
>       X18.5 X19.5 X20.5 X21.5 X22.5 X23.5 X24.5 X25.5 X26.5 X27.5 X28.5 
>        9.82    10 10.35  10.7 11.03 11.37 11.61 11.84 12.12 12.39 12.67
> 
>       X29.5 X30.5 X31.5 X32.5
>        12.96 13.28 13.47 13.65

You probably read SD24 from some source by read.delim which suppose the 
first line is header and subsequent lines (in your case only one) are 
data.

I would try to read it with header = FALSE option. You shall get data 
frame with 2 lines and you could then use

t(your.reed.data)

to transpose it to more convenient form.

Or you can use
x <- your.read.data[1,]
y <- your.read.data[2,]

to get x, y.

Regards
Petr

> R always considers the original data as different variables(as matrix).
> I already tried: y<-as.vector(SD24)
> It doesn't work.   SD24 is the object giving me the original data you 
see.
> 
> 
> sorry for writing so long; hope it is clear what I wrote.
> 
> Thanks for your efforts,   Benedikt
> 
> --
> 
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