Measuring the Occurrence of Disease

1
Measuring the Occurrence of Disease

• Chapters 3 4

2
Measures of Disease Frequency

• Measures of morbidity
• Measures of mortality

3
Prevalence

• The proportion of people who have a certain
  attribute either at a particular point in time or
  within a specific period of time.
• of people with disease X
• total population size
• Point prevalence
• Period prevalence

4
Prevalence

• In 1997, there were 1,854 cases of syphilis in
  Oak County.
• What should be used for the denominator?
• During 6 months, 3 states reported 55 cases of
  lyme disease.
• What should be used for the denominator?

5
Prevalence

• D
• It is the status of disease in a population.
• E
• Projects health care needs of affected
  individuals.

6
Prevalence Example

• At a screening of 1,000 men and women in the year
  2000, 200 were found to have high cholesterol.
• What is the prevalence of hypercholesterolemia in
  the population?
• 200 cases of hypercholesterolemia at screening
  in 2000
• 1,000 people in the population at screening in
  2000
• 200/1,000 .20 x 100 20
• Interpretation 20 of the population tested had
  hypercholesterolemia at the screening in 2000.

7
Incidence

• Number of new cases of a disease that occur
  during a specified period of time in a population
  at risk for developing the disease
• of new cases of a disease during
• specified time period
• of persons at risk during
• that period of time

8
Incidence

• The rate of developing a disease during a given
  period of time.

9
Incidence Example

• In Florida in 1999, there were 174 reported cases
  of measles. The total population of Florida in
  July of 1999 was 12,128,370.
• For every 100,000 persons living in Florida, what
  was the risk of measles in 1999?

10
Incidence Example

• Risk of Measles, Florida, 1999

11
Mortality Rates

• C
• death rate in entire population (U.S., state,
  county)
• S
• age specific, cause specific, gender specific,
  etc.
• C
• rate at which death occurs from the disease among
  those with the disease. (severity of disease)

12
Mortality Rates

• I
• death rate for children lt1 year of age per number
  of live births.
• N
• rate of death for children less than 28 days old
  per number of live births.

13
Strata Specific Rates

• Presenting event rates by categories - grouped by
  a particular trait.
• Provides a community diagnosis for further
  investigation.
• Also provides a basis for determining the health
  status of the population.

14
Strata Specific Rate Example

• Total population of Florida 12,128,370
• Population aged 65 and older 1,434,380
• Total deaths due to pneumonia influenza in
  Florida, 1998 3,944
• Total deaths due to pneumonia influenza among
  persons aged 65 and older, FL, 1998 3,496
• What is the age specific death rate for pneumonia
  influenza among persons aged 65 and older, FL,
  1998?

15
Strata Specific Rate Example

• deaths due to PI in persons aged 65 and older
• total population of those aged 65 and older
  in FL

16
Standardized Mortality Ratio

• A way to determine if the actual number
  (observed) of deaths in a population is greater
  than, equal to, or less than expected.
• SMR
• of observed deaths
• of expected deaths

17
SMR Example

• The number of deaths among persons who exercise
  regularly compared to expected number of deaths
  based on the general population.
• SMR
• of observed deaths 100
• of expected deaths 1,000
• SMR

18
SMR Interpretation

• SMR 1.0 means there is no increase in observed
  deaths in a population over what we would expect
  to find even when we adjust for age.
• SMR gt 1.0 means that the actual observed number
  of deaths in a population is greater than
  expected.
• SMR lt 1.0 means that the actual observed number
  of deaths in a population is less than expected.

19
Proportionate Mortality Ratio

• Number of deaths due to a specific disease
  proportionate to all deaths in a population.
• Relative importance of a particular disease.
• Useful for health care planning and indicating
  areas for further study.

20
PMR Example

• Calculate the PMR for heart disease in Florida
  based on the following 1998 statistics
• Total population of FL 12,128,370
• Deaths due to all causes in FL 104,070
• Deaths due to heart disease in FL 32,722
• Total deaths in U.S. 2,337,256
• Total population of U.S. 270,463,688

21
PMR Example

• PMR for heart disease in Florida, 1998
• deaths due to disease X in FL 1998
• total deaths in Florida in 1998