[R] extracting the t-statistic: just the numbers, please

From: Michael Rennie <mrennie_at_utm.utoronto.ca>
Date: Thu 29 Jul 2004 - 15:57:02 EST

Hi, there

I am quite sure there is an easy answer to this, but I am unsure how to gather a bunch of t-statistics in an organized format.

I am trying to generate a list of t-statistics for a randomization routine. If I try to collect a bunch of t-statistics from a run, this is what happens:

> M <- 10 ; simt <- NULL
> for(i in 1:M)
+ {
+ perm<-sample(site,replace=F)
+
+ permute<-cbind(perm, site, a, b, c)
+
+ m<- order(perm)
+
+ m1<-cbind(perm[m], site[m], a[m], b[m], c[m])
+
+ black<-c((m1[1:5,5]),(m1[11:15,5]))
+ #black
+
+ white<-c((m1[6:10,5]),(m1[16:20,5]))
+ #white
+
+ sims <- t.test(black,white,var.equal=FALSE,mu=0)$statistic
+ simt<-c(simt,sims)
+ #simt
+ } # Next i (next simulation)

>
> simt

         t t t t t t t  0.3474150 0.1542973 -0.4044992 1.2466663 -0.2933944 -0.5809257 0.7799080

         t t t
-1.4132713 1.2048335 -0.6596936

Which gives me a list, but not in a form that I can do anything with. This is in stark contrast to what happens when requesting p-values, which gives output like this:

 M <- 10 ; simt <- NULL
> for(i in 1:M)
+ {
+ perm<-sample(site,replace=F)
+
+ permute<-cbind(perm, site, a, b, c)
+
+ m<- order(perm)
+
+ m1<-cbind(perm[m], site[m], a[m], b[m], c[m])
+
+ black<-c((m1[1:5,5]),(m1[11:15,5]))
+ #black
+
+ white<-c((m1[6:10,29]),(m1[16:20,5]))
+ #white
+
+ sims <- t.test(black,white,var.equal=FALSE,mu=0)$p.value
+ simt<-c(simt,sims)
+ #simt
+ } # Next i (next simulation)

>
> simt
 [1] 0.6763749 0.7480091 0.9447851 0.3342029 0.7852635 0.3199006 0.5272153  [8] 0.3863616 0.7333693 0.7268907

Now THAT'S what I'd like to get for my t-statistics- a nice vector (simt) that I can deal with later, rather than the output I am currently getting (the first output above).

Does anyone know a way to extract JUST the t-statistics from the t.test, without the "t" character header, so I can generate a nice little vector? Alternatively, can I manipulate the output I am currently getting for the tstatistics  so that I can isolate just the numbers?

-- 
Michael Rennie
Ph.D. Candidate
University of Toronto at Mississauga
3359 Mississauga Rd. N.
Mississauga ON  L5L 1C6
Ph: 905-828-5452  Fax: 905-828-3792

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Received on Thu Jul 29 16:05:27 2004

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