[R] extracting the t-statistic: just the numbers, please
Michael Rennie
mrennie at utm.utoronto.ca
Thu Jul 29 07:57:02 CEST 2004
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 t-
statistics 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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