R help - Apr 2008 - means and variances of several groups in the matrix

Dear R users, 

I have 32 observations in data x.  After sorting this, I want to compute
means and variances of 3 groups divided by "nr".

Actually, the number of groups is flexible.  Any suggestion will be greatly
appreciated. 

Kathryn Lord

---------------------------------------------------------------------------
x=rnorm(32)
y=sort(x)

nr=matrix(c(12,11,10,10,10,11),2,3)
> nr
     [,1] [,2] [,3]
[1,]   12   10   10    -> sum=32
[2,]   11   10   11    -> sum=32

For the 1st row in "nr", index of y = (1,..,12,   13,...,23,  
24,...32)

I want to compute means and variances for 3 groups 

(1st group is 1 through 12; 2nd group is 13-23; 3rd group is 24-32)


For the 2nd row in "nr", index of y = (1,..,11,   12,...,22,  
23,...32)

also, I want to compute means and variances for 3 groups 

(1st group is 1 through 11; 2nd group is 12-22; 3rd group is 23-32)



-- 
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kathie wrote:
> Dear R users, 
> 
> I have 32 observations in data x.  After sorting this, I want to compute
> means and variances of 3 groups divided by "nr".
> 
> Actually, the number of groups is flexible.  Any suggestion will be greatly
> appreciated. 
> 
Hi Kathryn,
One way (there are many others) is to use the brkdn function in the 
prettyR package. You have to create a grouping variable, but it's pretty 
easy...

group1<-rep(1:3,each=nr[1,])
group2<-rep(1:3,each=nr[2,])
y.df<-data.frame(y=y,group1=group1,group2=group2)
brkdn(y~group1,y.df,num.desc=c("mean","var"))
brkdn(y~group2,y.df,num.desc=c("mean","var"))

Jim

Hi

r-help-bounces at r-project.org napsal dne 21.04.2008 09:03:30:
> 
> Dear R users, 
> 
> I have 32 observations in data x.  After sorting this, I want to compute
> means and variances of 3 groups divided by "nr".
> 
> Actually, the number of groups is flexible.  Any suggestion will be 
greatly
> appreciated. 
> 
> Kathryn Lord
> 
> 
---------------------------------------------------------------------------
> x=rnorm(32)
> y=sort(x)
> 
> nr=matrix(c(12,11,10,10,10,11),2,3)
> > nr
>      [,1] [,2] [,3]
> [1,]   12   10   10    -> sum=32
> [2,]   11   10   11    -> sum=32
> 
> For the 1st row in "nr", index of y = (1,..,12,   13,...,23,  
24,...32)
> 
> I want to compute means and variances for 3 groups 
> 
> (1st group is 1 through 12; 2nd group is 13-23; 3rd group is 24-32)
> 
> 
> For the 2nd row in "nr", index of y = (1,..,11,   12,...,22,  
23,...32)
> 
> also, I want to compute means and variances for 3 groups 
> 
> (1st group is 1 through 11; 2nd group is 12-22; 3rd group is 23-32)
> 
If you know that your vector is sorted you can do

sapply(split(y,rep(1:3, times=nr[1,])), mean)

Regards
Petr

> 
> 
> -- 
> View this message in context: 
http://www.nabble.com/means-and-variances-of-
> several-groups-in-the-matrix-tp16803939p16803939.html
> Sent from the R help mailing list archive at Nabble.com.
> 
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide 
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.

Hy Katie,

There are many ways to do this.  A simple one is to create a vector of 
the same length than your 'x' vector, containing a group label.

 > group=rep(c(1,2,3),times=nr[1,])

Then you can use tapply to apply a function (in this case mean and 
variance) of the values of x within each group.

 > means=tapply(x,group,mean)
 > vars=tapply(x,group,var)

 > means
           1           2           3
-0.14711206  0.28314274 -0.07861427
 > vars
         1         2         3
1.4584971 0.3611996 0.6300624

Julian


kathie wrote:
> Dear R users, 
> 
> I have 32 observations in data x.  After sorting this, I want to compute
> means and variances of 3 groups divided by "nr".
> 
> Actually, the number of groups is flexible.  Any suggestion will be greatly
> appreciated. 
> 
> Kathryn Lord
> 
> ---------------------------------------------------------------------------
> x=rnorm(32)
> y=sort(x)
> 
> nr=matrix(c(12,11,10,10,10,11),2,3)
>> nr
>      [,1] [,2] [,3]
> [1,]   12   10   10    -> sum=32
> [2,]   11   10   11    -> sum=32
> 
> For the 1st row in "nr", index of y = (1,..,12,   13,...,23,  
24,...32)
> 
> I want to compute means and variances for 3 groups 
> 
> (1st group is 1 through 12; 2nd group is 13-23; 3rd group is 24-32)
> 
> 
> For the 2nd row in "nr", index of y = (1,..,11,   12,...,22,  
23,...32)
> 
> also, I want to compute means and variances for 3 groups 
> 
> (1st group is 1 through 11; 2nd group is 12-22; 3rd group is 23-32)
> 
> 
>