measures of morbidity. morbidity and mortality weekly reports

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Measures of Morbidity

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Page 1: Measures of Morbidity.  Morbidity and Mortality Weekly Reports

Measures of Morbidity

Page 2: Measures of Morbidity.  Morbidity and Mortality Weekly Reports

www.cdc.gov/mmwr

Morbidity and Mortality Weekly Reports

Page 3: Measures of Morbidity.  Morbidity and Mortality Weekly Reports

Terms Related to Morbidity• Morbidity

The extent of illness, injury or disability in a defined population

• Incidence of a disease (Incidence rate)The number of new cases of a disease that

occur during a specified time period (numerator) in a population at risk for developing the disease (denominator)

• Prevalence of a disease (Prevalence rate)The number of total cases of disease present

at a particular time (numerator) in a specific population (denominator)

• RiskThe likelihood that an individual will contract a

disease

Page 4: Measures of Morbidity.  Morbidity and Mortality Weekly Reports

Characteristics

RISK PREVALENCE INCIDENCE RATE

Probability of disease

% of pop. with the disease

Rapidity of disease

occurrence

No units No units Cases per person-time

Newly diagnosed

Existing Newly diagnosed

“Cumulative incidence”

“Incidence density”

Page 5: Measures of Morbidity.  Morbidity and Mortality Weekly Reports

Problems with Numerators• Who has the disease?

• Who to include in numerator?

• Interview errors

Page 6: Measures of Morbidity.  Morbidity and Mortality Weekly Reports

Problems with Denominator

• Selective undercounting

• Everyone in denominator must have potential to enter numerator group

Page 7: Measures of Morbidity.  Morbidity and Mortality Weekly Reports

Problems with Hospital Data

• Selective (many reasons)

• Data may be unavailable, etc

Page 8: Measures of Morbidity.  Morbidity and Mortality Weekly Reports

IncidenceThe two forms of incidence are:

• Cumulative incidence• "risk of disease“• measures the proportion of persons who develop a disease in a

known span of time• Incidence rate

• "rate of disease“• measures the rate of new disease

occurrence over time

Page 9: Measures of Morbidity.  Morbidity and Mortality Weekly Reports

Incidence per 1000

What’s wrong with this formula?

Page 10: Measures of Morbidity.  Morbidity and Mortality Weekly Reports

Incidence Rate

• Measures the rapidity with which newly diagnosed cases of the disease of interest develop observe a population count # of new cases measure net time

• individuals in population at risk of developing disease

• person-time person-years patient-days

Page 11: Measures of Morbidity.  Morbidity and Mortality Weekly Reports

Incidence Rate (Attack Rate) (cont.)

• Can be used for specific exposures

• Also used for multiple exposures

• Other terms:primary casesecondary attack

• secondary cases

Page 12: Measures of Morbidity.  Morbidity and Mortality Weekly Reports
Page 13: Measures of Morbidity.  Morbidity and Mortality Weekly Reports

Principles of Incidence Rate

• Estimate probability

• Incidence kinetics

• Seasonal variation

• Place

• Person

Page 14: Measures of Morbidity.  Morbidity and Mortality Weekly Reports

Attack rates

Page 15: Measures of Morbidity.  Morbidity and Mortality Weekly Reports

Incidence and Attack Rates

• Primary Attack rates

Page 16: Measures of Morbidity.  Morbidity and Mortality Weekly Reports

Incidence and Attack rates(cont’d)

• Secondary Attack rates

Page 17: Measures of Morbidity.  Morbidity and Mortality Weekly Reports

Incidence RatesOSHA

Page 18: Measures of Morbidity.  Morbidity and Mortality Weekly Reports

Prevalence• Measure of the number (or proportion) of

cases in a given population• What is the difference between prevalence

and incidence?Prevalence a slice thru a population at a

given point in time that determines who has the disease and who does not, while Incidence only looks at new cases

• In steady state situation (no change in rate or net population)Prevalence = Incidence X Duration of disease

Page 19: Measures of Morbidity.  Morbidity and Mortality Weekly Reports

Prevalence

• Point prevalence- point in time

• Period prevalence- during a defined range of time

Page 20: Measures of Morbidity.  Morbidity and Mortality Weekly Reports

Comparative Factors Affecting Prevalence Rates

• Rates are INCREASED by Immigration of ill cases Emigration of healthy persons Immigration of susceptible cases or

those with potential of becoming cases Prolongation of life cases w/o cure

• Increase of duration of diseaseIncrease in occurrence of new cases

• Increase in incidence

Page 21: Measures of Morbidity.  Morbidity and Mortality Weekly Reports

Comparative Factors Affecting Prevalence Rates

• Rates are DECREASED by Immigration of healthy persons Emigration of ill cases Improved cure rate of cases Increased death rates from diseases Decrease in occurrence of new cases Shorter duration of disease Death

Page 22: Measures of Morbidity.  Morbidity and Mortality Weekly Reports

Prevalence Rates

Page 23: Measures of Morbidity.  Morbidity and Mortality Weekly Reports

Prevalence Rates

• Point

• Period

(cont’d)

Page 24: Measures of Morbidity.  Morbidity and Mortality Weekly Reports
Page 25: Measures of Morbidity.  Morbidity and Mortality Weekly Reports

Rates of MorbidityBreast Cancer Incidence in White Women and

Distribution by Age

Note change in slope – menopause??

Page 26: Measures of Morbidity.  Morbidity and Mortality Weekly Reports

Case DistributionSpot Map of Residence Distribution of Rheumatic Fever Cases in Baltimore

1960-1964

1977-1981

Page 27: Measures of Morbidity.  Morbidity and Mortality Weekly Reports

Sources of Morbidity Statistics• Clinical and hospital• Managed care• Registries• Vital statistics• Surveys• Disease reporting• Insurance and pre-paid med. care plans• Absenteeism records

Page 28: Measures of Morbidity.  Morbidity and Mortality Weekly Reports
Page 29: Measures of Morbidity.  Morbidity and Mortality Weekly Reports