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DM: QUERY: CHAID & KnowledgeSeeker questions
From: Tony Bagnall
Date: Wed, 18 Aug 1999 08:47:01 -0400 (EDT)
I was hoping someone could answer a few related questions about CHAID and KS
1. CHAID: if final groupings from two or more predictors are found to be significant, how does CHAID choose between them? I couldn't extract this info for the Kaas 1980 paper (although it may be there). My guess would be it chooses the predictor with the lowest p value, but this isnt completely obvious, since if for example a predictor has a p of 0.0004 and a Bonferroni adjusted threshold of 0.02, is it worse than a predictor with a p of 0.00035 and a Bonferroni adjusted threshold of 0.0004, or should the p values by adjusted by the Bonferroni as well as the significance levels?
2. KS: Again the question relates to how to choose between predictors for which a significant category grouping has been found. The Biggs et al 1991 paper says "the significance level at which the 'best' k-way split of the 'best' variable should be tested is ..." which implies to me the predictors are ranked by their p values then a level is calculated. However, comments in the manual made me doubt this and my initial assumption about CHAID: Page 166 manual "as the predictor variables are ranked according to their significance level, it is important that the calculated levels not favour one variable over another" which seems to be saying KS takes the (significant) predictor with the lowest upper bound as calculated by alpha/N_Bv*N_Bc (e.g. P_1 has p of 0.00035 and a Bonferroni adjusted threshold of 0.02, P_2 a p of 0.009 and a Bonferroni adjusted threshold of 0.01, choose P_2)
3. Another KS question about the adjusters. When in cluster mode, does it use the CHAID Bonferroni adjusters (as implied in the manual) or the KS adjuster? If it uses the CHAID adjusters, does anyone know why (especially after spending pages explaining how they can favour monotonic etc)?
thanks very much for any help, I appologise if the explanation is actually staring me in the faceTony Bagnall
(bogged down in the detail)
_______________________________________________________________ "There ain't no Devil, there's just God when he's drunk" -Tom Waits, Heartattack And Vine _______________________________________________________________