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Objectives-To review the appropriateness of the prevalence odds ratio (POR) and the prevalence ratio (PR) as eVect measures in the analysis of cross sectional data and to evaluate diVerent models for the multivariate estimation of the PR. Methods-A system of linear diVerential equations corresponding to a dynamic model of a cohort with a chronic disease was developed. At any point in time, a cross sectional analysis of the people then in the cohort provided a prevalence based measure of the eVect of exposure on disease. This formed the basis for exploring the relations between the POR, the PR, and the incidence rate ratio (IRR). Examples illustrate relations for various IRRs, prevalences, and diVerential exodus rates. Multivariate point and interval estimation of the PR by logistic regression is illustrated and compared with the results from proportional hazards regression (PH) and generalised linear modelling (GLM).Results-The POR is diYcult to interpret without making restrictive assumptions and the POR and PR may lead to different conclusions with regard to confounding and eVect modification. The PR is always conservative relative to the IRR and, if PR>1, the POR is always >PR. In a fixed cohort and with an adverse exposure, the POR is always >IRR, but in a dynamic cohort with suYcient underlying follow up the POR may overestimate or underestimate the IRR, depending on the duration of follow up. Logistic regression models provide point and interval estimates of the PR (and POR) but may be intractable in the presence of many covariates. Proportional hazards and generalised linear models provide statistical methods directed specifically at the PR, but the interval estimation in the case of PH is conservative and the GLM procedure may require constrained estimation. Conclusions-The PR is conservative, consistent, and interpretable relative to the IRR and should be used in preference to the POR. Multivariate estimation of the PR should be executed by means of generalised linear models or, conservatively, by proportional hazards regression. (Occup Environ Med 1998;55:272-277)

When multiple diagnostic tests are performed on an individual or multiple disease markers are available it may be possible to combine the information to diagnose disease. We consider how to choose linear combinations of markers in order to optimize diagnostic accuracy. The accuracy index to be maximized is the area or partial area under the receiver operating characteristic (ROC) curve. We propose a distribution-free rank-based approach for optimizing the area under the ROC curve and compare it with logistic regression and with classic linear discriminant analysis (LDA). It has been shown that the latter method optimizes the area under the ROC curve when test results have a multivariate normal distribution for diseased and non-diseased populations. Simulation studies suggest that the proposed non-parametric method is efficient when data are multivariate normal.The distribution-free method is generalized to a smooth distribution-free approach to: (i) accommodate some reasonable smoothness assumptions; (ii) incorporate covariate effects; and (iii) yield optimized partial areas under the ROC curve. This latter feature is particularly important since it allows one to focus on a region of the ROC curve which is of most relevance to clinical practice. Neither logistic regression nor LDA necessarily maximize partial areas. The approaches are illustrated on two cancer datasets, one involving serum antigen markers for pancreatic cancer and the other involving longitudinal prostate specific antigen data.

Abstract-The potential benefits and risks of physical activity before and during pregnancy are not well studied. We studied the relation between recreational physical activity and the risk of preeclampsia in a case-control study of 201 preeclamptic and 383 normotensive pregnant women. Participants provided information about the type, intensity, frequency, and duration of physical activity performed during the first 20 weeks of pregnancy and during the year before pregnancy. Women who engaged in any regular physical activity during early pregnancy, compared with inactive women, experienced a 35% reduced risk of preeclampsia (odds ratio, 0.65; 95% confidence interval [CI], 0.43 to 0.99).Compared with inactive women, those engaged in light or moderate activities (ie, activities with metabolic-equivalent scores Ͻ6) experienced a 24% reduced risk of preeclampsia (95% CI, 0.48 to 1.20). The corresponding reduction for women participating in vigorous activities (metabolic equivalent scores Ն6) was 54% (95% CI, 0.27 to 0.79). Brisk walking (average walking pace Ն3 mi/h), when compared with no walking at all, was associated with a 30% to 33% reduction in preeclampsia risk. Stair climbing was inversely associated with the risk of preeclampsia (P for trendϭ0.039). Recreational physical activity performed during the year before pregnancy was associated with similar reductions in preeclampsia risk. These data suggest that regular physical activity, particularly when performed during the year before pregnancy and during early pregnancy, is associated with a reduced risk of preeclampsia. Key Words: exercise Ⅲ pregnancy Ⅲ preeclampsia Ⅲ hypertension, pregnancy H ypertensive disorders during pregnancy are the second leading cause, after embolism, of maternal mortality in the United States, accounting for Ϸ15% of such deaths. 1 Hypertension in pregnancy is associated with complications, including abruptio placentae, cerebral hemorrhage, hepatic failure, and acute renal failure. 2 Preeclampsia, 1 of the hypertensive disorders, occurs in 3% to 4% of pregnancies and contributes to perinatal morbidity and mortality. Little is known about the occurrence of preeclampsia in relation to modifiable risk factors, such as recreational physical activity. Although the health benefits of recreational physical activity, such as reductions in the risk of essential hypertension, coronary heart disease, and type 2 diabetes are well documented, 3 surprisingly little is known about the relation between maternal physical activity and the risk of hypertensive disorders of pregnancy. More than a decade ago, Marcoux et al, 4 in their case-control study of Canadian women, reported that women who regularly participated in recreational physical activity during the first 20 weeks of pregnancy experienced a 43% reduction in risk of preeclampsia compared with sedentary women. The authors did not study the risk of preeclampsia in relation to maternal physical activity before pregnancy, nor did they evaluate the impact of typical daily activities, such as walki...

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