Unformatted text preview: umes that the relative risk associated with ‘any laceration’ is equivalent when comparing 4° versus none
to 1° – 3° (combined), 3° – 4° (combined) versus none
plus 1° – 2° (combined), and so on. In constrast, the CR
model assumes that the relative risk associated with
‘any laceration’ is equivalent to 4° versus 3°, 3° – 4°
(combined) versus 2°, and so on. The likelihood ratio
test of H0: β = 0 is rejected both for the PO and the CR
models, implying that midline episiotomy is a strong
predictor of lacerations during pregnancy, although
both models violated the proportional odds and parallel
slopes assumptions (discussed later).
The results of fitting a partial proportional odds model
to the laceration data are summarized in Table 5a.
Based on the fit of an unconstrained model (model 6),
the estimated log odds ratio comparing women with
‘any laceration’ to no laceration in relation to midline
episiotomy is β (0.7125), whereas the log odds comparing 2° – 4° to none plus 1°, 3° – 4° to none pl...
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- Spring '11
- Regression Analysis, Logit, proportional odds, Ordinal Model