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

[Christian Schulz]

>	data(sleep)
>      t.test(extra ~ group, data = sleep)$statistic[1]
        t
-1.860813
>      t.test(extra ~ group, data = sleep)$statistic[[1]]
[1] -1.860813

hope this helps,
christian


Am Donnerstag, 29. Juli 2004 07:57 schrieb Michael Rennie:
> 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?