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Unformatted text preview: seline category (i.e. women
with no laceration). DISCUSSION
Choice of an Ordinal Model
The choice between the cumulative logit and
continuationratio models merit further discussion.
Armstrong and Sloan15 argue that when the cumulative
logit model is valid implying that the cumulative logodds ratios are a constant, say β*, then the continuationratio model will begin at βj = β*, but will approach 0 as
j increases. This argument has led to the cumulative
logit model being proposed as an adjunct to the Cox’s
proportional hazards model13 i.e. continuationratio
model for survival data when hazard rates of groups are
thought likely to converge with time (see McCullagh4
and McCullagh and Nelder14 for a more thorough
discussion).
The choice of a simple logistic model to an ordered
response has its own disadvantages. Based on extensive
simulations, Armstrong and Sloan15 conclude that the
logistic model attains only between 50 and 75% efficiency, relative to the cumulative logit model for a fivelevel ordered response. Moreover, efficiency...
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 Spring '11

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