[R] Using t tests

[Jim Lemon]

On 07/08/2011 07:22 PM, Gwanmesia at aol.com wrote:
> Dear Sir,
>
> I am doing some work on a population of patients. About half of them are
> admitted into hospital with albumin levels less than 33. The other half have
> albumin levels greater than 33, so I stratify them into 2 groups, x and y
> respectively.
>
> I suspect that the average length of stay in hospital for the group of
> patients (x) with albumin levels less than 33 is greater than those  with
> albumin levels greater than 33 (y).
>
> What command function do I use (assuming that I will be using the chi
> square test) to show that the length of stay in hospital of those in group x is
> statistically significantly different from those in group y?
>
Hi Ivo,
Just to make things even more complicated for you, Mark's suggestion 
that the length_of_stay measure is unlikely to be normally distributed 
might lead you to look into a non-parametric test like the Wilcoxon (aka 
Mann-Whitney in your case) test. You will have to split your 
length_of_stay measure into two like this (assume your data frame is 
named "losdf"):

albumin_hilo <- albumin > 33
wilcox.test(losdf$length-of-stay[albumin_hilo],
  losdf$length_of_stay[!albumin_hilo])

or if you use wilcox_test in the "coin package:

albumin_hilo <- albumin > 33
wilcox_test(length_of_stay~albumin_hilo,losdf)

Do remember that the chi-square test is used for categorical variables, 
for instance if you dichotomized your length_of_stay into less than 10 
days or 10 days and over.

Jim