DefinePK

Abstract

Appying Type 1 and Type 2 Errors to Populations


Following on from our discussion in Part I1, let us now visit the vexing problem of Type 1 errors when the indicator used is endemic in the population. In recent years, epidemiologists have demonstrated the widespread presence in apparently "well populations" of findings associated with increased risk of disease, commonly recognized examples being elevated blood pressure and cholesterol levels. When individuals present with elevations in these measures, clinical importance is attached to them. But what are the implications on a population based level? A classic analysis conducted by the late Geoffrey Rose, on the topic of "sick individuals and sick populations", revealed theoretically that there is more to be gained by shifting the entire population curve to the left than to attend to the small proportion of individuals at ostensibly higher risk above a pre-determined cutpoint on the curve (Figure).2 This approach is supported by randomized trials that demonstrate a reduction in individual risk by lowering either blood pressure or cholesterol at virtually any point on the curve. In this example (in the clinical decision making context), lack of action based on an individual's value being below a clinical cutpoint is similar to a Type 1 error (where the null hypothesis is taken as disease is present): the individual is considered "well" and remains untreated when in fact some measure of intervention could have a beneficial impact. When taken across an adult population, the error magnifies and indeed accounts for the majority of related cardiovascular disease. [(0)]Adapted from Rose G.Slick indivudials and sick populations.International Journal of Epidemiology 1985,14:32-38____________________________________________________________________________________ Figure.Simulated distributions for"Healthy" and Diseased populations;BP and cholestrol From the above, it seems logical therefore to direct primary intervention efforts towards entire populations for particular risk factors than to solely target medical interventions to those deemed to be at "high risk". It may even be argued that there is more to be gained by doing away with individual testing and simply treating entire populations presumptively, especially in developing countries. In fact, the recommended daily use of low dose aspirin from middle age onwards is an example of such a mass action, even in the absence of individual risk assessment. In advocating this, we are committing a Type 2 error: there will be those taking such prophylaxis.Nonetheless, like fluoridation, this illustrates in the context of chronic diseases a rationale for public health action: to apply population approaches more emphatically, and not to focus so exclusively on medical diagnosis and treatment, both of which (by definition) come too late.In case such a proposal seems too radical, or somehow not applicable to Asia, a recent editorial in the New England Journal of Medicine notes that: "recent estimates from populations in east Asia suggest that a reduction of just 3 percent in average blood pressure levels in such populations (as might be achieved, for example, by sustained reductions in dietary sodium or caloric intake) would be expected to reduce the incidence of disease (largely among non-hypertensive persons) almost as much as would hypertensive therapy targeted to all hypertensive persons in the population".3