Denis
Many thanks for your reply, which was EXACTLY the response I needed. You have clarified
some important issues for me !
Thank you.
Martin
Martin J Watson
Senior Lecturer
Occupational Therapy and Physiotherapy (OPT)
School of Health (HEA)
University of East Anglia (UEA)
Norwich NR4 7TJ UK
Telephone: (44) 01603 593092
Fax: (44) 01603 593166
Email: [log in to unmask]
http://optcb.hsw.uea.ac.uk/welcome.html
On Mon, 5 Oct 1998 14:56:57 +0000 [log in to unmask] wrote:
> From:[log in to unmask]> Date: Mon, 5 Oct 1998 14:56:57 +0000
> Subject: Re: chi-squared test - query for the statistically inclined who
> To: [log in to unmask]
>
>
>
>
> Martin your query is a common one. Analysing percentages in a 2x2
> contingency table is used in initial exploratory work. The %
> difference across rows is used to gain insight into a possible
> relationship between variables. The use of percentages helps clarify
> the figures.
>
> In confirmatory analysis the chi-squared test is used to determine
> formally if any relationship is due to chance or not i.e.
> statistically significant. The test uses actual counts. Using
> percentages or any other method of transformation is incorrect.
>
> Look what happens to the figures when they are percentaged across
> rows.
>
> Original figures (% scores)
> total
> 19 (50%) 19 (50%) 38 (100%)
> 22 (58%) 16 (42%) 38 (100%)
>
> total 41 35
>
> The relationship between the figures within each row is maintained.
>
> 19 is 50% of 38.
> 22 is 58% of 38
> 16 is 42% of 38
> However, the relationship between the figures in each column has been
> lost.
>
> 19 is not 50% of 41
> 22 is not 58% of 41
> 19 is not 50% of 35
> 16 is not 42% of 35.
>
> The chi-square is computed using row totals and column totals and if
> these are not derived from the original data then the test will give
> a misleading result.
>
>
> Hope this helps
>
>
> Denis Martin
> Dept Physiotherapy
> Queen Margaret College
> Edinburgh
>
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