Health and Community Services Department
HEALTH SURVEILLANCE DATA BOOK
EXPLANATION OF DATA TERMS
The words used to describe health statistics can often be confusing or unclear. The following is a list of terms and their definitions that are commonly used in data reports provided by the Kalamazoo County Health and Community Services.
AGE-ADJUSTMENT: Age adjustment is used to compare risks of two or more populations at one point in time or one population at two or more points in time. Age-adjusted rates should be viewed as relative indexes rather than actual measures of risk. Age-adjusted rates are computed by the direct method by applying age-specific rates in a population of interest to a standardized age distribution, in order to eliminate differences in observed rates that result from age differences in population composition.
AGE-SPECIFIC RATE: Age-specific rates are calculated by dividing the number of deaths in a specific age group by the total number of persons in that age group. These rates are useful when you are interested in health events (cases, conditions, or deaths) occurring among a certain age group (example: birth rate among females aged 15-19).
ATTACK RATE: The proportion of individuals who are exposed to an infectious agent who become clinically ill. If some of the exposed individuals already are immune to the disease, they are excluded when calculating the attack rate.
CASE FATALITY RATE: The proportion of people who will die of a certain disease out of those who contract it. A time frame is referenced in the case fatality rate (example: case fatality rate for measles is often measured as those who die within 4-6 weeks after the rash appears). This measure also can be applied to chronic conditions; in reference to chronic disease the converse of the case fatality rate, survival rate, is calculated (see Survival Rate).
CAUSE OF DEATH RANKING: Selected causes of death of public health and medical importance comprise tabulation lists and are ranked according to the number of deaths assigned to these causes. The top-ranking causes determine the leading causes of death. Certain causes on the tabulation lists are not ranked if, for example, the category title represents a group title (such as Major cardiovascular diseases and symptoms, signs, and abnormal clinical and laboratory findings, not elsewhere classified); or the category title begins with the words "Other" and "All other." In addition when one of the titles that represents a subtotal (such as Malignant neoplasms) is ranked, its component parts are not ranked.
CRUDE MORTALITY RATE: The death rate in an entire population, calculated without age-adjusting the rate to the standard population (see Age-Adjustment and Mortality Rate).
ICD CODES: The International Classification of Diseases provides the ground rules for coding and classifying cause-of-death data. Diseases listed on death certificates are assigned specific codes according to this system. The ICD is developed collaboratively between the World Health Organization (WHO) and 10 international centers. This system allows mortality (death) data to be collected and compared among different areas. Since the beginning of the century, the ICD has been modified about once every 10 years in order to stay abreast with advances in medical science. New revisions usually introduce major disruptions in time series of mortality statistics due to changes in classification and rules for selecting underlying cause of death. Classification and rule changes impact cause-of-death trend data by shifting deaths away from some cause-of-death categories and into others. Comparability ratios measure the effect of changes in classification and coding rules.
INCIDENCE: Incidence is the number of cases of disease having their onset during a prescribed period of time. It is often expressed as a rate (for example, the incidence of measles per 1,000 children 5-15 years of age during a specified year). Incidence is a measure of morbidity or other events that occur within a specified period of time. Only NEW cases are counted as incident cases.
INFANT MORTALITY RATE: Calculated by dividing the number of infant deaths during a calendar year by the number of live births reported in the same year. It is expressed as the number of infant deaths per 1,000 live births.
MATERNAL MORTALITY RATE: The number of maternal deaths per 100,000 live births. The maternal mortality rate is a measure of the likelihood that a pregnant woman will die from maternal causes. The number of live births used in the denominator is a proxy (substitute or approximation) for the population of pregnant women who are at risk of a maternal death.
MEAN: The average (very sensitive to extreme/outlying values).
MEDIAN: The middle of the values where half of the values fall above and ½ of the values fall below (the 50th percentile).
MORTALITY RATE: The measure indicating what proportion of the population die from the disease each year. Death rates are expressed as the number of deaths per 100,000 population. The rate may be restricted to deaths in specific age, race, sex, or geographic groups (specific rate) or it might be age-adjusted to a standard population. Alternatively, it may be related to the entire population and be unadjusted for the distribution of ages in the population (crude rate). A disease or condition may have a high case fatality rate yet have a low mortality rate; even if most people who contract the disease die from it, if relatively few people in the population contract the disease the mortality rate will be low.
PREVALENCE: Prevalence is the number of cases of a disease, infected persons, or persons with some other attribute present during a particular interval of time. It is often expressed as a rate (for example, the prevalence of diabetes per 1,000 persons during a year). Prevalence is influenced by the incidence (number of new cases), the duration of disease (how long the disease lasts), and the case fatality/survival rate (the proportion of people who have the disease that will die from it).
RATE: The number of events occurring in a time period is influenced by the size of the population. A large population has more health events simply due to its larger size. A rate is a measure of an event, disease, or condition in relation to a unit of population, along with some specification of time, and it allows you to compare events in population of different sizes. It is calculated by taking the number of events divided by the total population and is typically multiplied by a standard value (1,000, 10,000 or 100,000) and is expressed as "rate per 100,000 population," for example. Rates based on small numbers are considered to be unreliable (see 95% Confidence Interval and Three-Year Moving Average Rate for additional details).
RELATIVE RISK: Relative risk is the ratio of the risk of disease/health event among a group of people who have been exposed and the risk of disease/health event among people who have not been exposed. It gives you an idea of how strongly a health event may be associated with an exposure. If the ratio of risk is greater than one, it is possible that the exposure is associated with the disease/event. The "exposure" does not need to be an agent of disease (such as a virus or bacteria) or a risky behavior (such as smoking); for example, we can calculate the relative risk of a certain disease, behavior or outcome occurring among males vs. females (using being female as the exposure). (Example: the one-year incidence of chlamydia among males aged 15-19 is 1,035.3 per 100,000; among females in this age group the incidence is 3,604.3 per 100,000. The relative risk is 3.5, which tells us that female teens are approximately 3.5 times more likely to get chlamydia than male teens.)
SURVIVAL RATE: "Survival Rate" refers to the proportion of people who are still alive after a certain period of time after a chronic disease is diagnosed. Survival rate is often expressed as relative survival rate, which takes into account the expected probability of survival in the absence of the particular disease for a specific group. Relative survival is calculated as the observed survival in people with the disease divided by the expected survival rate if the disease were absent. Survival from acute diseases is expressed in terms of fatalities among cases (see Case Fatality Rate).
THREE-YEAR MOVING AVERAGE RATE: Rates based on a small number of events are combined into a single rate that covers a longer time period in order to make rates more reliable. For a three-year moving average rate, groupings of years overlap to create a smoothed rate so that trends over time can be observed (example: rates are calculated for years 1997-1999, 1998-2000, 1999-2001, 2000-2002, etc.).
UNDERLYING CAUSE OF DEATH: For the purpose of national mortality statistics, every death is attributed to one underlying condition, based on information reported on the death certificate and using the international rules for selecting the underlying cause of death from the conditions stated on the death certificate. The underlying cause is defined by the World Health Organization (WHO) as the disease or injury that initiated the train of events leading directly to death, or the circumstances of the accident or violence that produced the fatal injury.
YEARS OF POTENTIAL LIFE LOST (YPLL): Years of Potential Life Lost (YPLL) is a measure of premature mortality. YPLL is presented for persons under 75 years of age because the average life expectancy in the United States is over 75 years. YPLL-75 is calculated using the following eight age groups: under 1 year, 1-14 years, 15-24 years, 25-34 years, 35-44 years, 45-54 years, 55-64 years, 65-74 years. The number of deaths for each age group is multiplied by the years of life lost, calculated as the difference between age 75 years and the midpoint of the age group. For the eight age groups the midpoints are 0.5, 7.5, 19.5, 29.5, 39.5, 49.5, 59.5, and 69.5. For example, the death of a person 15-24 years of age counts as 55.5 years of life lost. Years of Potential Life Lost is derived by summing years of life lost over all age groups.
95% CONFIDENCE INTERVAL: This refers to the range of values within which the true prevalence or percentage lies with a specified degree of assurance. This range is influence by the number of events and by the size of the sample. The lower confidence limit (LCL) represents the low number of this range and the upper confidence limit (UCL) represents the high number in the range.
Example: What does a rate of 260.0 +/- 12.6 per 100,000 population mean?
Answer: We are 95% confident that the true rate lies between the LCL of 247.4 and the UCL of 272.6 per 100,000 population.
Tip: If the confidence interval is larger than the rate, the results are not reliable (example: 10.2 +/- 23.6 is not a reliable rate).