[R] Correlating data after changing one observation for one variable

[arun]

Hi,

trees <-  structure(list(Girth = c(8.3, 8.6, 8.8, 10.5, 10.7, 10.8, 11, 
11, 11.1, 11.2, 11.3, 11.4, 11.4, 11.7, 12, 12.9, 12.9, 13.3, 
13.7, 13.8, 14, 14.2, 14.5, 16, 16.3, 17.3, 17.5, 17.9, 18, 18, 
20.6), Height = c(70L, 65L, 63L, 72L, 81L, 83L, 66L, 75L, 80L, 
75L, 79L, 76L, 76L, 69L, 75L, 74L, 85L, 86L, 71L, 64L, 78L, 80L, 
74L, 72L, 77L, 81L, 82L, 80L, 80L, 80L, 87L), Volume = c(10.3, 
10.3, 10.2, 16.4, 18.8, 19.7, 15.6, 18.2, 22.6, 19.9, 24.2, 21, 
21.4, 21.3, 19.1, 22.2, 33.8, 27.4, 25.7, 24.9, 34.5, 31.7, 36.3, 
38.3, 42.6, 55.4, 55.7, 58.3, 51.5, 51, 77)), .Names = c("Girth", 
"Height", "Volume"), row.names = c("1", "2", "3", "4", "5", "6", 
"7", "8", "9", "10", "11", "12", "13", "14", "15", "16", "17", 
"18", "19", "20", "21", "22", "23", "24", "25", "26", "27", "28", 
"29", "30", "31"), class = "data.frame")


with(trees,cor(Volume,Height))
#[1] 0.5982497
 with(trees,cor(Volume,Girth))
#[1] 0.9671194

 trees$Volume[31]<- 35


with(trees,cor(Volume,Girth))
#[1] 0.9012941
 with(trees,cor(Volume,Height))
[1] 0.5298093


#Looks like you ?attach() trees


attach(trees)

 cor(Volume,Height)
#[1] 0.5298093
 cor(Volume,Girth)
#[1] 0.9012941



trees[31,3] <- 77


You need to attach() again
attach(trees)

 cor(Volume,Height)
#[1] 0.5982497
 cor(Volume,Girth)
#[1] 0.9671194


It is better not to use ?attach().  Try using ?with()


A.K.


After having changed the last observation for Volume from 77.0 to 35, 

> trees[31,3]<-35 
> trees 
   Girth Height Volume 
1    8.3     70   10.3 
2    8.6     65   10.3 
3    8.8     63   10.2 
4   10.5     72   16.4 
5   10.7     81   18.8 
6   10.8     83   19.7 
7   11.0     66   15.6 
8   11.0     75   18.2 
9   11.1     80   22.6 
10  11.2     75   19.9 
11  11.3     79   24.2 
12  11.4     76   21.0 
13  11.4     76   21.4 
14  11.7     69   21.3 
15  12.0     75   19.1 
16  12.9     74   22.2 
17  12.9     85   33.8 
18  13.3     86   27.4 
19  13.7     71   25.7 
20  13.8     64   24.9 
21  14.0     78   34.5 
22  14.2     80   31.7 
23  14.5     74   36.3 
24  16.0     72   38.3 
25  16.3     77   42.6 
26  17.3     81   55.4 
27  17.5     82   55.7 
28  17.9     80   58.3 
29  18.0     80   51.5 
30  18.0     80   51.0 
31  20.6     87   35.0 

the correlation coefficient does not change, which it should... 
earlier: 
> cor(Volume, Girth) 
[1] 0.9671194 
> cor(Volume, Height) 
[1] 0.5982497 

and after changing value: 
> cor(Volume, Girth) 
[1] 0.9671194 
> cor(Volume, Height) 
[1] 0.5982497 

Why is this? 
Thanks in advance ;) 
Victoria