Intro to Epi chapter 5

Crude Rate Formula

# deaths/ Total population size

Problems comparing crude rates among populations:

-Groups differ with respect to underlying characteristics that affect overall rate of disease (for example, age) and so you may be making an unfair comparison -Things may be more "different" than "same"

Crude Rate Ratio

1.3 X greater in Sweden compared to Panama

Crude Rate Difference =

2 per 1,000 greater in Sweden

Crude Rate Difference?

2 per 1,000 greater in Sweden than in Panama

Direct Adjustment, cont. Apply the weights to create age-adjusted mortality rates:

Adjusted mortality rates -- what the populations of interest would have experienced if they each had the same age-distribution as the standard population. Summary rate that accounts for age difference between populations. Any differences between age-adjusted rates cannot be attributed to age.

For a fair comparison what do you need?

Age adjustment

Comparing Rates in Two or More Populations:Age-adjustment can be used for rates, risks, or prevalence.

Age-adjustment can be used for rates, risks, or prevalence.

Direct Adjustment, cont.:Method overview:

Compute a weighted average of the age-specific rates. Weights: the age distribution of a specified standard population.

Race Comparison of All-Cause Mortality Rates among Females, U.S., 1998

Crude mortality rate for whites = 903.7 deaths per 100,000 pop per year Crude mortality rate for blacks = 746.4 deaths per 100,000 pop per year

Rate Comparison:

Crude rate because NOT taking any other population characteristics (that might affect the event rate) into account

Direct Adjustment

Formula shows that the directly adjusted rate is a weighted average of strata-specific rates

An age-adjusted rate for a single population is not a meaningful quantity

Hypothetical; applies to no one Age-adjustment only takes on meaning when 2 or more populations or groups are being compared

Direct Age-Adjustment: Reference populations arbitrary; may be:

One of the populations under study Average of all populations under study Age distribution from some external place at some time ("standard million") Sometimes the population in a census year is used.

Why the apparent paradox?

Pop. A mostly young, Pop. B mostly old... You're OK as long as you compare like-to-like

Category specific rates

Rates specific to some particular sub-population: age-specific, race-specific, sex-specific

Comments on Age Adjusted Rates

Standard population is arbitrary Does not measure the rate in the standard population

Wise comparisons

Statement: 'I had 10 calves die of respiratory disease last week' Given: There are 20 calves in the herd 'That's terrible!' Given: 40,000 calves in the herd 'Congratulations!

Comparing Rates in Two or More Populations: How do we get around this problem of differences in age structures?

The difference in the age structure between the 2 populations of black women and white women make this an unfair comparison: We can compare age-specific rates, but this will be cumbersome.

what is an example of an standard population

The world population

Age Distribution of White and Black Females, 1998

They are different: black population has a younger age distribution.

for consideration

To compare levels of disease among groups of individuals, time frames or locations we need to consider counts of events or cases in context of the size of the population from which those cases arose

What is the age adjustment or Standardization of Rates

We want the summary # for both populations that allows for comparison with each other accounting for differences in age

conclusion:Simplest method to make

a fair comparison is to restrict to like-to-like comparisons (strata- specific comparisons)

direct age adjustment

a standard population is used in order to eliminate the effects of any differences in age between two or more populations being compared

Crude rates:

a summary measure calculated by dividing the total number of cases in the population by the total number of individuals in that population at a specified time period

what does age confound

age confounds the association between mortality and country

Comparing Rates in Two or More Populations:Calculate a summary rate for each population:

age-adjusted or age-standardized rate uses a procedure designed to minimize the effects of differences in age composition

rate ratio (RR)

also called relative risk, compares the risk of a health event (disease, injury, risk factor, or death) among one group with the risk among another group. It does so by dividing the risk (incidence proportion, attack rate) in group 1 by the risk (incidence proportion, attack rate) in group 2.

For simplicity, let "rate"

any incidence or prevalence measurement

What formula to you use to calculate mortality Rate

by using crude rate

conclusion:Strata-specific information

can be recombined in a fair way using direct or indirect "standardization" -EPID 6210 covers direct......take EPID 6211 for indirect!

Conclusion: Crude rates

cannot be used when making comparisons

what cant we use crude rates for

comparison

Standard Populations

only for comparison purpose, its artificial: U.S. population in 1940, 1970, 2000, 2010 Soviet census in 1989; Russian census in 2002 Planned census of U.S. (2010) and Russia (2010) World population European population Combined populations (if convenient)

crude rate

over all summary ( the young and old put together)

Crude rate (synonym: unadjusted rate)

rate for entire population

Strata-specific rate

rate within a strata (subgroup)

What is age adjust rate formula

standard weight X Mortality

Direct Standardization Remember: the three things

standardized rates are artificial, not a real reflection of what is going on in an individual community to add communities to the comparison process, you MUST use the same reference population you cannot compare your standardized results with anyone else, unless the same reference population was used by

Direct Standardization In order to compute a standardized rate by the direct method you need:

stratum-specific rates a standard or reference population structure

direct adjustment

the single population that you want to study

what is standard population

when you force two populations to have the same age distribution

Comments on Age Adjusted Rates: Allows us to compare mortality rates across populations how ?

with different age structures: if each population had the age distribution of the standard population, how would their rates compare

Adjusted rate (synonym: standardized rate)

≡ mathematically compensated rate