# Special Types of Frequency Measures

Prevalence and incidence are the fundamental measures of disease frequency, but special names have evolved for these measures, depending on their specific use. All of these tend to be referred to as rates, even though, strictly speaking, they often refer to proportions (cumulative incidence or prevalence).

## Category-specific Rates

Either prevalence or incidence can be broken down into categoies, e.g., age groups, or by gender, or race, or some combination of these. For example, since disease frequency often differs substantially with age, one frequently sees "age-specific" rates of disease.

Example 1: A table of age-specific rates of stroke (incidence)

Age Group

# New Occurrences

Group Size

Cumulative Incidence

per 100,000 persons

0-34

0

582,083

0

35-44

28

113,581

25

45-54

114

114,208

100

55-64

320

91,484

350

65-74

550

81,155

900

75+

1.126

37,531

3,000

Example 2: A Table of race-specific causes of death per 100,000 population (mortality rates, i.e., incidence) in the US, 1967

White

Black

Hypertension

21.1

68.6

Homicide

3.5

32.3

Diabetes mellitus

16.6

28.9

Tuberculosis

2.5

9.6

Suicide

11.3

5.7

Leukemia

7.4

5.5

Syphilis

1.0

3.0

## Special Measures of Incidence

Morbidity rate is the incidence of non-fatal cases of a disease in a population during a specified time period. For example, during 1982 there were 25,520 non-fatal cases of TB in the US population. The mid-year population was estimated at 231,534,000. Therefore, the

Morbidity rate of TB =25,520/231,534,000  = 11.0/100,000 over one year

Note that this is a cumulative incidence and therefore is really a proportion, not a true rate.)

Mortality Rate: In 1982 there were 1,807 deaths from TB in the US population, so the mortality rate for TB was 7.8 per million over one year (also a cumulative incidence, not a true rate).

Case-Fatality Rate: the number of deaths from a specific disease divided by the total number of cases of that disease, i.e. the proportion of fatal cases of a disease (%). This provides a measure of the severity of the disease.

Example: Reyes Syndrome is a rare, but highly fatal disease in which the liver and brain become dysfunctional due to abnormal accumulation of cellular fat. It tends to occur when people are recovering from a viral illness, and it tends to be associated with use of aspirin, especially in children. If there were 200 cases of Reyes syndrome in 1982 and 70 died, then the case-fatality rate would be 70/200 = 35% over one year.

[Note: This is generally calculated by dividing the deaths reported in a given year by the number of cases reported in the same year, but this can be misleading since some diseases (e.g., TB) aren't rapidly fatal. Thus, many of the TB fatalities that occurred in 1982 were due to cases diagnosed several years earlier.]

Attack Rate: a cumulative incidence for a disease during a specific period (e.g., an epidemic).

Example: After a church picnic in Oswego, NY many attendees got food poisoning. There were 75 people at the picnic; 46 got sick within several hours, so the attack rate was 46/75 = 61%.

Live birth rate: the frequency of live births in one year per 1,000 females of childbearing age.

Infant Mortality Rate: the frequency of deaths in children under 1 year of age occurring during a one year period per 1,000 live births.

## Special Prevalence Measures

These are often incorrectly referred to as incidences or rates, but they are, in fact, proportions..

Autopsy Rate: the proportion of people who have a particular finding on a postmortem exam (the prevalence of a certain finding among the population of people who get autopsied).

Birth Defect Rate: the prevalence of a congenital abnormality at the "point" of birth. The denominator can be either live births or total births (which includes live births + stillbirths), but it generally does not include spontaneously aborted fetuses.