# st: RE: What does contribution to chi-square mean in tabchii

 From "Nick Cox" <[email protected]> To <[email protected]> Subject st: RE: What does contribution to chi-square mean in tabchii Date Sat, 29 Nov 2003 11:35:31 -0000

```-tabchii- is a user-written command. A version
which will work in Stata 6 through 8 is
available from the -tab_chi- package on SSC
and an earlier version which will work in
Stata 5 is available from
http://www.stata.com/users/njc/tab_chi/

I am not sure what kind of answer Don
wants here. The definition of contribution
given in the help file is

(observed frequency - expected frequency)^2
/ expected frequency

and Don clearly understands that. The
programmer's intent in allowing
such contributions to be shown was that
some users like to see that step, perhaps
even as a check on hand calculations.
Personally I much prefer to look at
the (signed) square root of that

(observed freq - expected freq)
/ sqrt(expected frequency)

given the usual argument that the direction of
deviation from expectation is often helpful
in interpretation. These quantities are called Pearson
residuals in -tabchii- and indeed
elsewhere. And in turn a refinement
has a distribution in the null case
that is closer to Gaussian(0,1).

Nick
[email protected]

> Can you please tell me how to interpret the chi-square
> 'contribution' to
> chi-sqare as used in tabchii.  I find it useful in telling
> me where the
> total chi-square is coming from but is there more that one
> can get out of
> it? i.e. say the chi-square (total) is 100 and the
> contribution of one cell
> is 55, another is 35, another 5 and another 5.  Obviously
> the first cell's
> chi square counts a lot but can I say anything else, say in
> regard to cell
> 2's chi-square of 35.  Or is it just an interesting way of
> getting an
> insight into the data.

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