Re: st: testing for significant differences between independent samples using svytab

[Ronan Conroy <rconroy@rcsi.ie>]

On 28 Jan 2008, at 19:43, Lacey Hartman wrote:

> I am wondering why I get different results when using these 2  
> approaches to testing for a significant difference between two  
> independent samples.  I am also wondering which approach is more  
> accurate and why.
>
> #1 compare confidence intervals generated from svytab row obs ci se
> I find that the confidence intervals overlap
>
> #2
> test for significant difference using the following  t-test for  
> independent samples:
>
> value2-value1/ square root of (standard error of value 1 ^2  
> +standard error of value 2^2)
>
> this formula gets me to a value of 2.12, significant at the 95% level
>
> Again, why is there a difference and which approach is more correct?
You are confusing the confidence intervals for the means with the  
confidence interval for the difference. The t-test is based on the  
difference between the means and its standard error, and so is a  
first cousin of the confidence interval for the difference between  
the means.

There are going to be cases where the confidence intervals for the  
individual means overlap, but the confidence interval for the  
difference does not overlap zero.


P    Before printing, think about the environment
=================================
Ronan Conroy
rconroy@rcsi.ie
Royal College of Surgeons in Ireland
Epidemiology Department,
120 St Stephen's Green, Dublin 2, Ireland
+353 (0)1 402 2431
+353 (0)87 799 97 95
http://www.flickr.com/photos/ronanconroy/sets/72157601895416740/

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