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

 From Ronan Conroy <[email protected]> To [email protected] Subject Re: st: testing for significant differences between independent samples using svytab Date Tue, 29 Jan 2008 16:12:05 +0000

```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
[email protected]
Royal College of Surgeons in Ireland
Epidemiology Department,
120 St Stephen's Green, Dublin 2, Ireland
+353 (0)1 402 2431
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http://www.flickr.com/photos/ronanconroy/sets/72157601895416740/

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