[R] T-test syntax question

Vito Ricci vito_ricci at yahoo.com
Wed Nov 24 09:33:42 CET 2004


You did not specify if data are paired or not, as data
are paired you should use option paired=TRUE in
t.test(). Variances of the two samples have to be not
significatevely different, (see ? var.test) to use
t.test, if not you should specify var.equal=FALSE.

var.equal: a logical variable indicating whether to
treat the two ariances as being equal. If 'TRUE' then
the pooled variance is used to estimate the variance
otherwise the Welch (or Satterthwaite) approximation
to the degrees of freedom is used.

paired: a logical indicating whether you want a paired

If 'paired' is 'TRUE' then both 'x' and 'y' must be
specified and they must be the same length.  Missing
values are removed (in pairs if 'paired' is 'TRUE'). 
If 'var.equal' is 'TRUE' then the pooled estimate of
the variance is used.  By default, if 'var.equal' is
'FALSE' then the variance is estimated separately for
both groups and the Welch modification to the degrees
of freedom is used.
>From the output of your test you're sending I
understand that variances of the two samples are
significatively different (Welch Two Sample t-test)
and delta values are also significatively different
from 0.

See ? t.test


You wrote:


I'd like to do a t-test to compare the Delta values of
items with Crit=1 with Delta values of items with
Crit=0. What is the t.test syntax?

It should produce a result like this below (I can't
get in touch with the person who originally did this
for me)

    Welch Two Sample t-test

data:  t1$Delta by Crit
t = -3.4105, df = 8.674, p-value = 0.008173
alternative hypothesis: true
difference in means is not equal to 0
95 percent confidence interval:
 -0.04506155 -0.00899827
sample estimates:
mean in group FALSE  mean in group TRUE 
         0.03331391          0.06034382 


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