2. MEASURES OF DISEASE FREQUENCY AND STANDARDIZATION Flashcards
What is the flow chart structure for epidemiologic measures?
- Count
- Ratio
a. proportion
- prevalance
b. rate
- Incidence
- crude
- specific
- adjusted
What is a Ratio?
The value obtained by dividing one quantity by another
The most general form has no specified relationship between the numerator and denominator
What epidemiologic measure do rate, proportion, and percentages fall under?
Ratios
How do you calculate the ratio?
Of 1,000 motorcycle fatalities, 950 victims are men and 50 are women
950/50 = 19
19:1 male to female
What is a proportion?
A measure that states a count relative to the size of the group.
A ratio in which the numerator is part of the denominator
may be expressed as a %
What is the usefulness of a proportion?
It can demonstrate the magnitude of a problem
What is this an example of:
10 dormitory students develop hepatitis. If only 20 students live in the dorm, 50% are ill and if 500 students live in the dorm, only 2% are ill.
A proportion
How do you calculate the proportion when out of 4,960 boys aged 5 to 14 years, there were 1,150 African-American deaths and 3,810 White deaths
- A+B or 3,810 + 1,150 = 4,960
- A/(A+B) or
1,150/4,960 x 100 = 23.2%
What is a rate?
A ratio that consists of a numerator and a denominator and in which time forms part of the denominator
What contains the following elements?
- Disease frequency
- Unit size of population
- Time period during which an event occurs
A rate
What are populations at risk?
The population at risk is the population that is exposed, for example, the total population in the case of deaths or the legally married population in the case of divorces
Does any proportion adjust for group size (or population size)?
yes
What are measures of disease frequency?
Prevalence
Incidence Proportion (risk)
Incidence Rate (incidence density)
Mortality
What is prevalence?
It is the number of existing cases of a disease or health condition in a population at some designated time
New + old
What is the interpretation of prevalence?
prevalence provides an indication of the extent of a health problem
like the prevalence of diarrhea at a camp
What is the prevalence of diabetes when a random sample of 1,000 individuals from a population demonstrates 52 diabetics and 948 non-diabetics?
52/(52+948) = 0.052 or 5.2%
What are the uses for prevalence?
When describing the burden of a health problem in a population
When estimating the frequency of an exposure
When determining allocation of health resources such as facilities and personnel
What is the equation for point prevalence?
# of individuals with the condition at a point in time ----------------------------------------- # of individuals considered at a point in time
What is this an example of - during the course of a semester 23 of the 58 students in a class experienced at least one upper respiratory infection
And what is the calculation?
Period prevalence because it’s over a certain point in time (a semester)
23/58 = 39.7%
What is the definition of Incidence?
The number of new cases of a disease that occur in a group during a certain time period
only new!
What else can incidence proportion (risk) be called?
Cumulative Incidence
What is Incidence Proportion (risk)?
It describes the rate of development of a disease in a group over a certain time period
What are the 3 elements to incidence proportions (risk)?
- Numerator - the # of new cases
- Denominator = the population at risk
- Time = the period during which the cases occur
What is this an example of - A cohort on individuals at risk for uterine cancer is followed for 5 years during which time 45 subjects develop the disease
Incidence proportion
What is the alternate form of incidence proportion?
Attack rate
What is attack rate used for?
for diseases observed in a population for a short time period
Is attack rate a true rate?
no, because the time is often uncertain
like a disease outbreak period
What is the equation for attack rate?
sick/sick + well x 100 (during a time period)
When is incidence density (rate) used?
An incidence measure is used when members of a population or study group are under observation for different lengths of time
What is the equation for incidence density (rate)?
Total person-time (years) of observation
What is incidence density (rate)?
a measurement of the rate at which new cases of disease occur in the population at risk during a defined period
ranges from 0 to infinity
What is the interrelationship equation between prevalence and incidence?
The prevalence (p) of a disease is proportional to the incidence rate (I) times the duration (D) of the disease or P = I x D
What is the interrelationship between prevalence and incidence when there is a short duration?
If duration of disease is short and incidence is high, prevalence becomes similar to incidence because with short duration cases either recover quickly or are fatal
example: the flu
What is the interrelationship between prevalence and incidence when there is a long duration?
If duration of disease is long and incidence is low, prevalence increases greatly relative to incidence.
example: HIV/AIDS
What is this an example of - from 1973 to 1977, the average annual incidence rate of lung cancer was 45.9 per 100,000 and the average annual prevalence was 23.0 per 100,000.
The interrelationship between prevalence and incidence
What does mortality measure?
it measures the severity of disease in a population. It qualifies the risk of having a relatively shorter life span
Why is mortality important given that everyone dies anyway?
It is important because it helps determine whether treatment has become more effective over time
Can serve as surrogate for incidence rates of lethal diseases where identifying new cases is difficult
How is mortality used?
It is used to compare severity across groups and different populations
How is mortality rate like incidence?
It can be calculated using mid-point of the population of interest in denominator (risk) or with person-years in denominator (true rate)
what are some specific mortality rates used to describe differences across population characteristics?
cause specific, gender specific, age specific, and etc.
What is the equation for specific mortality?
# of deaths with specific... -------------------------------------- # of people specific at mid-year
What is the equation for proportionate mortality?
Total # of deaths from all causes
What is Natality?
The ratio of the number of births to the size of the population; birth rate
What is crude birth rate used for?
It is used to project population changes
How are crude birth rates affected?
by the number and age composition of women of childbearing age
What is the equation for crude birth rate?
population size at the middle of that period
X
1,000
per 1,000
Should you use crude death rates with caution? Why?
yes, because observed differences in crude rates may be the result of systematic factor (sex or age distributions) within the population rather than true variation in rates
What are specific rates?
refers to a particular subgroup of the population defined in terms of race, age, sex, or single cause of death or illness
What are adjusted/standardized rates?
The summary measures of the rate of morbidity and mortality in a population where statistical procedures have been applied in order to remove the effect of differences in composition of various populations
When are direct methods used to adjust rates?
the direct method may be used if age-specific death rates in a population to be standardized are known and a suitable standard population is available
Why is there a direct standardization method?
To remove bias due to age
What are the 4 steps to use the direct standardization method?
- select a reference population
- multiply the age-specific disease rates of the comparison population by the age distribution of the reference population
- sum the expected # of cases of disease for the comparison population
- divide the total expected # of cases of disease for the comparison population by the total number of people in the reference population
What is the equation for age adjusted rates using direct standardization?
Total reference population
When are standardization rates meaningful?
they are only meaningful when directly compared to each other
Is the choice of the reference population in direct standardization important?
yes, should not be abnormal or unnatural
When would an indirect method be used?
If age-specific death rates of the population for standardization are unknown or unstable
example: the rates to be standardized are based on a small population
What is SMR
Standardized Mortality Ratio
When is SMR used?
can be used to evaluate the results of the indirect method
What is used to evaluate the indirect method?
SMR
What is the equation for SMR?
expected deaths
x
100
What does it indicate when the observed and expected #s are the same in SMR?
that observed mortality is not unusual
What does 2.0 mean in SMR?
that the death rate in the study population is tow times greater than expected
What are 3 common disease rates?
- Crude, or unadjusted
- category specific, or stratified
- adjusted, or standardized
What are crude disease rates?
they estimate the actual disease frequency for a population
What are category specific disease rates?
rates for subgroups of the total population
can provide general characteristics of the frequency of disease in a population, particularly by person, place, and time
Are crude rates or category specific rates more reliable?
category specific rates because they provide more detailed information about patterns of disease frequency in the population
What are the pro’s and con’s to category specific disease rates?
pro - can be used for valid comparison of populations
con - can be cumbersome if there is a large # of categories to compare
What is one of the most important factors to consider when describing the occurrence of any disease or illness?
Age
