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ment can leave more people with a condition surviving in the population. If more people survive with more health problems, it becomes difficult to know when one country is “healthier” than another.

At the moment, most of the data available for cross-national comparisons indicate the prevalence of health problems in the population. The prevalence of a health problem at a given time depends on how many people have experienced the onset of the problem or condition and how long they survived with the problem. The onset rate or incidence of a problem depends on risk for the condition in the exposed population, whereas the survival rate can depend on whether the case is treatable and, if treated, whether death or the progression of severity of disease is delayed. Populations can be in better health because the incidence of a disease is lower, but they could also have a lower prevalence of poor health if those with diseases did not survive as long. For instance, if life expectancy among the diseased and disabled increases, population health as measured by disability could deteriorate. Two countries with the same level of disease incidence but different approaches to treatment could have differences in population health; where disease is aggressively treated and death prevented, the level of disease prevalence as well as life expectancy could be higher. So the health status of a population depends on a set of processes of onset and survival that cannot be inferred from one or more snapshots of the prevalence of health problems in the population.

There can also be variation in the presence of diseases and conditions across countries and across time for a number of reasons. Diagnostic definitions can differ across countries and change over time. For instance, the blood pressure cutoff value indicating hypertension has gotten lower over time, so that diagnosis occurs at an earlier stage of severity in more recent years. Countries may adopt changes in definitions at different times, leading to variability of the definition of conditions at one time. Another example is differences in the diagnostic criteria for diabetes (DECODE Study Group, 1998; Wareham and O’Rahilly, 1998). Differences in national emphasis on screening for conditions can also affect variability in knowledge of the existence of diseases and reported prevalence. This is true for cancer, hypertension, high cholesterol, and diabetes (Ashworth, Medina, and Morgan, 2008; Gregg et al., 2004; Wareham and O’Rahilly, 1998). It is also possible that recognition of disease varies over time and across countries. For instance, Alzheimer’s disease (AD) is now a recognized cause of both morbidity and mortality but was virtually unknown and unrecognized in the 1950s. The timing of accepting AD as a cause of mortality and morbidity can differ across countries. It is also possible that there are national or cultural differences in the way doctors disclose conditions to patients (Asai, 1995).

There are multiple dimensions of health to be considered in evaluating national differences in health. Health change with age in populations begins

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