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Workingsci1.Qk Research Commentary Effect Measures in Prevalence Studies Neil Pearce Centre for Public Health Research, Massey University Wellington Campus, Wellington, New Zealand commonly used effect measure is the risk There is still considerable confusion and debate about the appropriate methods for analyzing preva- ratio, which is the ratio of the incidence pro- lence studies, and a number of recent papers have argued that prevalence ratios are the preferred portion in the exposed group (a/N1) to that in method and that prevalence odds ratios should not be used. These arguments assert that the preva- the nonexposed group (b/N0). In this example, lence ratio is obviously the better measure and the odds ratio is “unintelligible.” They have often the risk ratio is 0.1813/0.0952 = 1.90. A third been accompanied by demonstrations that when a disease is common the prevalence ratio and the possible effect measure is the incidence odds prevalence odds ratio may differ substantially. However, this does not tell us which measure is the ratio, which is the ratio of the incidence odds more valid to use. In fact, the prevalence odds ratio a) estimates the incidence rate ratio with fewer in the exposed group (a/c) to that in the non- assumptions than are required for the prevalence ratio; b) can be estimated using the same methods exposed group (b/d). In this example, the odds as for the odds ratio in case–control studies, namely, the Mantel–Haenszel method and logistic ratio is 0.2214/0.1052 = 2.11. regression; and c) provides practical, analytical, and theoretical consistency between analyses of a These three multiplicative effect measures prevalence study and prevalence case–control analyses based on the same study population. For are sometimes referred to under the generic these reasons, the prevalence odds ratio will continue to be one of the standard methods for analyz- term of relative risk. In this example, they all ing prevalence studies and prevalence case–control studies. Key words: epidemiology, methods, show that the rate (or risk, or odds) of develop- prevalence case–control studies, prevalence studies. Environ Health Perspect 112:1047–1050 ing the disease under study is about twice as (2004). doi:10.1289/ehp.6927 available via http://dx.doi.org/ [Online 18 March 2004] high in the exposed group as in the nonexposed group, but their precise estimates vary (2.00, 1.90, and 2.11, respectively). Thus, they are all Although the methods for analyzing incidence is the proportion of study subjects who expe- approximately equal when the disease is rare studies (and incidence case–control studies) are rience the outcome of interest at any time during the follow-up period (e.g., an incidence now well established, there is still considerable during the follow-up period. In this instance, proportion < 10%). However, although the confusion and debate about the appropriate there were 952 incident cases among the rate ratio and (to a lesser extent) the risk ratio methods for analyzing prevalence studies (and 10,000 people in the nonexposed group, and are both commonly used for analyzing inci- prevalence case–control studies). In particular, the incidence proportion, b/N0 = R0, was dence studies, the odds ratio has been severely it has been argued that prevalence ratios are therefore 952/10,000 = 0.0952 over the criticized as an effect measure (Greenland the preferred method and that prevalence 10-year follow-up period. When the outcome 1987; Miettinen and Cook 1981) and has lit- odds ratios (PORs) should not be used. In this of interest is rare over the follow-up period tle intrinsic meaning in incidence studies. article I argue that PORs should continue to (e.g., an incidence proportion < 10%), then the be one of the standard methods for analyzing incidence proportion is approximately equal to Prevalence Studies such studies. I briefly review the relationship the incidence rate multiplied by the length of Incidence studies are the ideal method for between incidence and prevalence studies and time that the population has been followed (in studying disease occurrence because they then discuss the relative merits of using PORs the example this product is 0.1000, whereas the involve collecting and analyzing all the rele- and prevalence ratios. incidence proportion is 0.0952). vant information on the source population, A third possible measure of disease occur- and we can get better information on when Incidence Studies rence is the incidence odds (Greenland 1987), exposure and disease occurred. However, Table 1 shows the findings of a hypothetical which is the ratio of the number of people who these types of studies involve lengthy periods incidence study of 20,000 persons followed experience the outcome (b) to the number of of follow-up and many resources in terms for 10 years (Pearce 2003). Three measures of people who do not experience the outcome (d). of both time and funding, and it may be dif- disease incidence are commonly used in inci- As for the incidence proportion, the incidence ficult to identify incident cases of nonfatal dence studies (Pearce 1993): the person-time odds is dimensionless, but it is necessary to chronic conditions such as diabetes or asthma. incidence rate, the incidence proportion, and specify the time period over which it is being Furthermore, in some instances we may be the incidence odds. These all involve the same measured. In this example, the incidence odds, more interested in factors that affect the cur- numerator: the number of incident cases of b/d = O0, is 952/9,048 = 0.1052. When the rent burden of disease in the population. disease (b). They differ in whether their outcome is rare over the follow-up period, the Consequently, although incidence studies are denominators represent person-years at risk incidence odds is approximately equal to the (Y0), persons at risk (N0), or survivors (d). incidence proportion. Address correspondence to N. Pearce, Centre for The person-time incidence rate is a meas- Corresponding to these three measures of Public Health Research, Massey University Wellington ure of the disease occurrence per unit popu- disease occurrence, there are three principal Campus, Private Box 756, Wellington, New Zealand. lation time and has the reciprocal of time as ratio measures of effect that can be used in Telephone: 64-4-380-0606. Fax: 64-4-380-0600. E-mail: [email protected] its dimension. In this example (Table 1), incidence studies (Pearce 1993): the rate ratio, I thank J. Douwes, S. Greenland, and A. ’t Mannetje there were 952 cases of disease diagnosed in the risk ratio, and the incidence odds ratio. for their comments on the draft manuscript, and the nonexposed group during the 10 years of The rate ratio is the ratio of the incidence D. Kriebel for useful discussions concerning these issues. follow-up, which involved a total of 95,163 rate in the exposed group (a/Y1) to that in The Centre for Public Health Research is supported by a Programme Grant from the Health Research person-years, and the person-time incidence the nonexposed group (b/Y0). In the example rate, b/Y = I , was 952/95,163 = 0.0100 (or in Table 1, the incidence rates are 0.02 per Council of New Zealand. 0 0 The author declares he has no competing financial 1,000 per 100,000 person-years). person-year in the exposed group and 0.01 interests. The incidence proportion, or average risk, per person-year in the nonexposed group, and Received 19 December 2003; accepted 18 March is a second measure of disease occurrence and the rate ratio is therefore 2.00. A second 2004. Environmental Health Perspectives • VOLUME 112 | NUMBER 10 | July 2004 1047 Commentary | Pearce usually preferable, there is also an important that the disease is rare and therefore (1 – P1) the incidence rate ratio with greater validity role for prevalence studies, for practical reasons and (1 – P0) are close to 1.0. than does the prevalence ratio. and because such studies enable the assessment Of course, such a steady-state population Prevalence case–control studies. Just as an of the level of morbidity and the population will rarely exist in practice, but it will be incidence case–control study can be used to “disease burden” for a nonfatal condition approximated in situations where disease inci- obtain the same findings as a full incidence (Pearce 2003; Thompson et al. 1998). dence and the relevant exposures are not study, a prevalence case–control study can be Measures of effect in prevalence studies. changing markedly over time (provided the used to obtain the same findings as a full Figure 1 shows the relationship between inci- other assumptions specified above are met). prevalence study in a more efficient manner. dence and prevalence of disease in a “steady- This is also conditional on other risk factors In particular, if obtaining exposure informa- state” population. Suppose we denote the (e.g., age) because even when incidence is tion is difficult or costly (e.g., if it involves prevalence of disease in the study population independent of age, prevalence will often be lengthy interviews, or serum samples), then it by P, and we assume that the population is in age dependent (Keiding 1991, 2000), and may be more efficient to conduct a prevalence a steady state (stationary) over time (in that these other risk factors therefore need to be case–control study by obtaining exposure the numbers within each subpopulation controlled for in the analysis. information on all of the prevalent cases and a defined by exposure, disease, and covariates do Table 2 shows data from a prevalence sample of controls selected at random from not change with time)—this usually requires study of 20,000 people, with the data derived the noncases.
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