MEASUREMENT OF HEALTH STATUS

" Health " is a multi-dimensional concept that is usually and measured in terms of: l) absence of physical pain, physical disability, or a condition that is likely to cause death, 2) emotional well-being, and 3) satisfactory social functioning. Some have a dvocated including the quality of an individual's physical environment in the definition of health, but this dimension is not at present included in the most widely used measures of health.

There is no single " standard " measurement of health status for individuals or population groups. Individual health status may be measured by an observer (e.g., a physician), who performs an examination and rates the individual along any of several dimensions, including presence or absence of life-threatening illness, risk factors for premature death, severity of disease, and overall health. Individual health status may also be assessed by asking the person to report his/her health perceptions in the domains of interest, such as physical functioning, emotional well-being, pain or discomfort, and overall perception of health. Although it is theoretically attractive to argue that the measurement of health should consist of the combination of both an objective component plus the individual's subjective impressions, no such measure has been developed.

The health of an entire population is determined by aggregating data collected on individuals. The health of an individual is easier to define than the health of a population. Once the definition of optimum health for the individual is agreed upon, health status can be placed along a continuum from perfect health to death. No comparable scale exists for whole populations. What is the population-level equivalent of death? (Keep in mind that it is unusual for entire populations to die.) What is the population-level equivalent of optimum health?

In the absence of comprehensive or absolute measures of the health of a population, the average lifespan, the prevalence of preventable diseases or deaths, and availability of health services serve as indicators of health status. Judgments regarding the level of health of a particular population are usually made by comparing one population to another, or by studying the trends in a health indicator within a population over time.

Some commonly used measures of population health status are:

Morbidity Measures

Incidence rate =  Number of new cases of a disease occurring in the population during a 
                  specified time period
                 _______________________________________________________________________
	          Number of persons exposed to risk of developing the disease during 
                  that period of time


Prevalence =  Number of cases of disease present in the population at a specified 
              period of time
             ________________________________________________________________________
	      Number of persons at risk of having the disease at that specified time

The above ratios are multiplied by 1,000 or 100,000 to yield statistics that are more readily interpretable. Click here for your Assignment"

Mortality Measures


Death Rate =  Number of deaths in the population during a specified time period
             ___________________________________________________________________
	      The number of persons in the population during the specified time 
              period

The denominator is usually defined as the number of persons in the population at the midpoint of the time period (usually 12 months). The rate is multiplied by 1,000 or 100,000 for ease of interpretation. Death rates, or mortality rates can be calculate d for deaths from specific causes, and for specific age and gender groupings.

Death rates can be calculated for all causes combined, specific causes, and particular age-sex groups.

In order to compare mortality rates across different population groups or time periods, the rates must be " standardized " to a population with the same age structure. For example, if you are interested in comparing mortality from colon cancer in Hispanics and non-Hispanics in the U.S. in 1970 and 1990, the " crude " death rates in the two populations at two different points in time will not be comparable. The Hispanic population is likely to be younger on average than the non-Hispanic population at both time points, and the median age of both populations can be expected to have increased over the 20 year time interval. Since the prevalence of colon cancer increases with age, unadjusted mortality rates would underestimate the prevalence in Hispanics at both points in time, and the prevalence would be underestimated for both populations in 1970 compared to 1990. In order to avoid errors in interpretation, mortality rates must be adjusted to a common population with a known age structure. The choice of standard population is arbitrary. When reviewing mortality statistics, always check the footnotes of tables for information on the reference population that was used to standardize the mortality rates.


                                 Table 1
             Death Rates for Diseases of the Heart in Persons 
                     45 Years and Over, 1988-1990

		
	         Deaths per 100,000 Resident Population

	

Ethnic Group	                    Age-Adjusted Rate(*)   Crude Rate


White	                                 553.6	             950.7

Black	                                 779.3	           1,031.5

Asian/Pacific Islander	                 290.1	             331.1

American Indian or Alaskan Native        393.5	             453.6

Hispanic	                         383.2	             461.9


(*)Age adjusted by the direct method to the U.S. population enumerated in 1940.
Source:  National Center for Health Statistics: Health United States 1992, Table 31.


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The age adjusted relative risk of heart disease death for whites compared to Hispanics is much lower than the crude relative risk. This reflects differences in the age structure of the two populations. Failure to adjust for this age difference would overestimate the differences in heart disease mortality between the two population groups.

Other Indicators

Infant mortality rate =	 Number of deaths to infants under age 1 X 1,000
                        _________________________________________________	 
                                       Total live births	
The infant mortality rate is a widely used indicator of a population's health status because it is associated with education, economic development, and availability of health services.
Life expectancy:
The average number of additional years a person can expect to live from a given age onward.

Life expectancy at birth is the statistic usually calculated for population groups. Life expectancy is calculated by apply age and sex-specific mortality rates from the population under study to a hypothetical birth cohort of 100,000 individuals. Life expectancy is a theoretical measure and can change for an individual with changing trends in disease frequency in the population and with individual behavioral changes. Lower life expectancy in developing countries is usually a result of high infant mortality. Once individuals reach adulthood, their life expectancy tends to be comparable across different population groups.

Table 2 contains recent average life expectancy estimates at birth, 1 year, 15 years, 45 years and 65 years for males in three different countries. Notice that males born in Mexico in 1989 can expect to live an average of 69.3 years at birth, compared to 74.1 years for Norwegians and 71.9 years for U.S. males. In what age group does it appear that the mortality experience of Mexican males results in a decrease in life expectancy at birth compared to males in the U.S. and Norway? What causes of death could account for these mortality differences?


                       TABLE 2

                        MALES			

     Life            Norway      U.S.      Mexico
  Expectancy	      1990       1990       1989
                
    Age 0	      74.1	 71.9	    69.3

    Age 1	      73.6	 71.7	    69.9

    Age 15	      59.8	 58.0	    56.7

    Age 45	      31.4	 30.8	    30.2

    Age 65	      14.9	 15.2	    15.0

Source:  World Health Organization: World Health 
         Statistics Annual, 1993.			

Assignment

Exercise 1

Use the following numbers to calculate the annual incidence rate and 1991 prevalence of AIDS per 100,000 population in Hispanics in Houston, Texas. For ease of calculation, assume that cases alive at the beginning of 1991 live for the entire year.

Total estimated Hispanic population in 1991: 452,780

Total cases of AIDS in Hispanics reported from 1981- 1990: 850

Total new cases of AIDS reported in Hispanics in 1991: 95

Total deaths from AIDS in Hispanics from 1981 (first year reporting began) to 1990: 595

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Exercise 2

Examine the age adjusted and crude death rates due to heart disease in 1988-1990 reported by the National Center for Health Statistics. What effect does age adjustment have on all of the rates? Which ethnic group has the highest age adjusted heart disease death rate? Which group the lowest?

A common way to compare the probability of death or disease in two groups is to calculate the ratio of the measures of disease frequency in the groups. This ratio is referred to as the " relative risk." For example, if the incidence rate of breast cancer in Hispanic women is 20/100,000 compared to 45/100,000 in Black women, the relative risk (RR) for breast cancer in Black women compared to Hispanic women is 2.25. Black women can be said to have a 225% excess risk of developing breast cancer than Hispanic women. Calculate the relative risk of heart disease death in whites compared to Hispanics using the figures in Table 1. Calculate the RR's using first the crude death rates, then the age adjusted death rates. What happens to the RR when you use the age adjusted death rates?

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