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Re: [apstat] work shown for chisquare
Posted:
Dec 19, 2008 12:59 PM


On Dec 19, 2008, at 12:11 PM, Skerbie wrote:
> What work is expected of students in a chisquare significance test? > Can they just write the following: > X^2 = sum((OE)^2/E) = 13.95, df = 12, p = 0.3039, etc., > or do they have to show each individual component? > > > Also, in checking the conditions, do they actually need to show a > twoway table of observed values and/or a twoway table of expected > values (particularly the latter), or can they just state "all > expected counts are greater than 5"?
Here, as in all such situations, students must SHOW (not just state) that they checked the conditions. That means (for example) 1) showing the histogram that supports the claim of approximate normality; 2) showing the scatterplot that supports the claim of approximate linearity; 3) showing the calculation that establishes np and nq are large enough; 4) showing the expected counts for chisq; and so on.
I think students could get away with showing just the formula and calculator result, but I always recommend they actually substitute the values as an insurance policy. When pushing a wrong button leads to the wrong calculated value, as a Reader I can't be sure the student really knew what was going on unless I see something convincing on the page. Every gory detail isn't necessary, but something like this (with just a couple of terms) really helps: X^2 = (12  11.37)^2/11.37 + (18  22.43)^2/22.43 + .... If that much is correct, I can forgive "answers" that may be wrong (but reasonable).
 Dave
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