HSS4303B – Intro to Epidemiology Jan 21, 2010 – Standardization.

HSS4303B – Intro to Epidemiology

Jan 21, 2010 – Standardization

Survival

Survival rates• _____________ is the probability of remaining alive

for a specific length of time• 1 year and 5 year survival are often used as

indicators of the severity of disease and the prognosis

• 5 year survival rates for myelocytic leukemia is about 0.14, indicating that about 14% of the patients with acute myelocytic leukemia survive for at least 5 years after diagnosis.

• Survival (S) = (A – D) / A where– A is the number of newly diagnosed patients under

observation and D is the number of deaths observed in a specified period of time

Observation of each patient begins at diagnosis (time = 0), and continues until one of the following outcomes occurs: death, survival for 5 years, or follow-up ceases (the subject is "censored"). A patient is censored when follow-up ends prior to death or completion of a full period of observation. Follow-up could end for one of several reasons: (1) the patient decides to discontinue participation, (2) the patient is "lost" to follow-up, or (3) the study ends.

Five of the six people under observation (N = 6) survive at least 2 years. Thus, the 2-year survival is 5/6= 0.83= 83%

Specifying Length of Survival

• 1, 2, 5 years are standard, but it can be anything

• For example, prostate cancer has a much higher one year overall survival rate than pancreatic cancer, and thus has a better prognosis.

Relative Survival Rate• RSR• Ratio of survival rate of disease in question, divided by

the survival rate of the general population• Eg RSR of cancer at 2 years = (% of cancer patients who are alive @ 2 years) / (% of general population who are alive @ 2 yrs)

• Why is this important?

Eg, The overall 5-year relative survival rate for 1999-2005 from 17 SEER geographic areas was 89.1%. Five-year relative survival rates by race were: 90.3% for white women; 77.9% for black women.

Cause-Specific Survival Rate

• Cause-specific survival (CSS) is a term that denotes the chances of death due to a particular condition (or cause) at a particular point of time. It takes care to exclude death due to unrelated causes in patients suffering from the cancer in question.

The 5-year cause-specific survival for stage IIA Hodgkin lymphoma is 85% when treated with ABVD followed by involved field radiation.

This means that 15% of these patients are estimated to die directly due to the Hodgkin disease by 5 years. The remaining 85% are either alive or have died due to other unrelated causes.

Problems with mortality data

• Information on mortality is obtained from death certificates

• Deaths are coded according to the underlying cause – Which is defined as the disease or injury which initiated

the train of morbid events leading directly or directly to death or circumstances of accident or violence which produced the fatal injury

– The underlying cause therefore excludes information pertaining to the immediate cause of death, contributory causes and those that intervene between the underlying and immediate causes of death

• Some causes of death have better validity than others

Cause of death from a death certificate

ICD classification on death certificates

• Deaths are coded by ICD classification • Drop in diabetes related deaths in 1949 were caused

by changes in classification codes• Prior to 1949 the policy was to include diabetes as

cause of death anywhere on the certificate lead to diabetes being mentioned on the death certificate

• After 1949 only death certificates on which the underlying cause of death was listed as diabetes were coded as a death from diabetes

Changes in death rates from diabetes caused by changes in classification

Changes in the definition of disease

• In 1993, a new definition of AIDS was introduced

• These changes resulted in a rapid rise in the number of reported cases

AIDS cases in the US from 1984-2000

Causes of death in the early 20th century

Table 4-6. Some Causes of Death That Were Reported on Death Certificates in the Early 1900s

"Died suddenly without the aid of a physician"

"A mother died in infancy"

"Deceased had never been fatally sick"

"Died suddenly, nothing serious"

"Went to bed feeling well, but woke up dead"

Standardization

Consider the following…

• There is concern that the nuclear power plant in Raywatville (which is a beachfront community in Florida) is causing cancer.

• Death rates due to cancer are computed and compared to death rates due to cancer in a similarly-sized control community –Gomesland– in Alaska.

• The rates in Raywatville are indeed higher than the rates in Gomesland

• What can you conclude?

What is this an example of?

Con…..

So What Can We Do To Eliminate the Uncertainty?

Standardization

• A way to make two populations more comparable by adjusting one or both of them to conform to an external standard– Age– Sex– etc

Age-Adjusted Rates

• if you want a measurement of mortality that can be used either to compare different populations (states, counties, cities, etc.) or to compare the mortality experience over time for one area with a changing population, it is advisable to adjust or standardize the effects of such factors as age and/or sex in these groups.– Age is the most commonly used adjustment factor– Thus age-adjusted rates are the most common form of

standardization

Standardization

• First step is to define a reference population, which serves as the “standard” against which the test populations will be set– Often select the national census– Sometimes select one of the two test populations

to be the standard

Two Kinds of Age Standardization

1. Direct

2. Indirect

http://www.paho.org/English/SHA/be_v23n3-standardization.htm

•estimates the rate that would have been observed if the study population had had the same age structure as the reference group

•computes the number of cases of disease that would have been expected if the disease rates from the reference population had applied in the study population.


1. Direct

2. Indirect

http://www.paho.org/English/SHA/be_v23n3-standardization.htm

•commonly used in reports of vital statistics (e.g., mortality) or disease incidence trends (e.g., cancer incidence). Invented in 1899.

•Commonly used in studies of occupational disease or studies of place and time-limited environmental catastrophes. Can be computed from SMR (standardized mortality ratio). Invented in 1844.

There are three major components that are needed to perform adjusted mortality rate calculations:

1.the number of deaths2.the population3.a "standard" population

Direct Standardization

Raywatville Gomesland Reference Pop.

The steps

• We are going to standardize each population (Raywatville and Gomesland) individually against the reference (standard) population

• Multiply the age-specific rate for the test population against the size of standard population in each age stratum

• Add ‘em all up and divide by the total number of people in the standard population

Directly Standardized Rate for Raywatville (Miami) is…

Directly Standardized Rate for Gomesland (Alaska) is…

Compare Age-Standardized Rates

• Raywatville = 6.92 deaths/ 1000 people• Gomesland = 6.71 deaths/ 1000 people


Before age-adjustment:

After age-adjustment:?

Some Thoughts on Direct Standardization

• Because we are multiplying age-specific rates by the age-specific populations in the standard population, the final age-adjusted rate is a weighted average, with the weights being the proportion of people in each age stratum of the standard population

• We have a term for each such product: expected – For direct standardization, the expected number of

deaths is what we get when we multiply the known rate by the standard population

– The observed number of deaths is the actual number of people who died

Choice of Standard Pop

• We chose the US national survey as an objective standard population

• What if we had simply chosen to adjust the Gomesland rates by applying the Raywatville population as a standard?

• Shall we try?

Raywatville Gomesland

Age group Raywatville population (weight)

Gomesland age-specific rate

Gomesland age-adjusted deaths

<1515-2425-4445-6465+

1143508025913344014267092168

1.590.91.136.0239

Age group Raywatville population (weight)

Gomesland age-specific rate

Gomesland age-adjusted deaths

<1515-2425-4445-6465+

114350 x80259 x133440 x 142670 x92168 x

1.59 =0.9 =1.13 =6.02 =39 =

181816.572233.1150787.2858873.43594552

TOTALS 562887 4858262.2

Directly standardized rate = 4858262.2 / 562887 = 8.63 deaths / 1000 pop

Compare Age-Standardized Rates



Before age-adjustment:

After age-adjustment:?

Your Homework Assignment #1

1. Compute Age-Adjusted Rates in the previous example, using Gomesland as the standard population, rather than Raywatville

2. Then compute Age-Adjusted Rates using the hybrid population of “Gomesland + Raywatville” as the standard population

3. How do your conclusions differ?

Homework Assignment #2

Early Period Later Period

PopulationNo. of Deaths

Death Rate per 100,000 Population

No. of Deaths

Death Rate per 100,000

900,000 862 900,000 1,130

Early Period Later Period

Age Group (yr) Population

No. of Deaths

Death Rates per 100,000 Population

No. of Deaths

Death Rates per 100,000

All ages 900,000 862 96 900,000 1,130 126

30-49 500,000 60 300,000 30

50-69 300,000 396 400,000 400

70+ 100,000 406 406 200,000 700 350

Compute age-adjusted mortality rates for the early and later stages of this hypothetical disease, using a combined population (early + later) as the standard population.


Age Group (yr)

Standard Pop

Age-specific Mexico

Mortality Rates per 100,000

Expected Numbers of

Deaths Using Mexico Rates

Age-specific United States

Mortality Rates per 100,000

Expected Numbers of Deaths Using

United States Rates

All ages

100,000

<1 2,400 1693.2 41 737.8 18

1-4 9,600 112.5 11 38.5 4

5-14 19,000 36.2 21.7

15-24 17,000 102.9 90.3

25-44 26,000 209.6 176.4

45-64 19,000 841.1 702.3

65+ 7,000 4,967.4 5,062.6

Total numbers of deaths expected in the standard population

Age-adjusted rates:

Mexico = ? Per 1000 population United States = ? Per 1000 population

Fill in the table and compue adge-adjusted rates for both Mexico and the USA, using the given standard population.

Indirect Age Standardization

• Involves the computation of a Standardized Mortality Ratios (SMR)

• SMR= observed number of deaths per yearexpected number of deaths per year

•For indirect standardization, the expected number of deaths is what we get when we multiply the standard rate by the sample population•The observed number of deaths is the actual number of people who died

SMR

• SMR is a ratio, therefore is given simply as a number or as a percentage, but has no units

• The ratio of observed to expected deaths

SMR for tuberculosisTable 4-13. Computation of a Standardized Mortality Ratio (SMR) for Tuberculosis, All

Forms (TBC), for White Miners Ages 20 to 59 Years, United States, 1950

Estimated Population for White Miners

Death Rate (per 100,000) for TBC in Males in the General

Population

Expected Deaths from TBC in White Miners if They Had the Same Risk as the

General Population

Observed Deaths from TBC in White Miners

Age (yr) (1) (2) (3) = (1) ×(2) (4)

20-24 74,598 12.26 9.14 10

25-29 85,077 16.12 13.71 20

30-34 80,845 21.54 22

35-44 148,870 33.96 98

45-54 102,649 56.82 174

55-59 42,494 75.23 112

Totals 534,533 181.09 436

SMR = observed deaths for an occupation – cause – group / expected deaths for occupation – cause – group x 100SMR = 436/181.09x100 = 241

SMR for tuberculosisTable 4-13. Computation of a Standardized Mortality Ratio (SMR) for Tuberculosis, All

Forms (TBC), for White Miners Ages 20 to 59 Years, United States, 1950

Estimated Population for White Miners

Death Rate (per 100,000) for TBC in Males in the General

Population

Expected Deaths from TBC in White Miners if They Had the Same Risk as the

General Population

Observed Deaths from TBC in White Miners

Age (yr) (1) (2) (3) = (1) ×(2) (4)

20-24 74,598 12.26 9.14 10

25-29 85,077 16.12 13.71 20

30-34 80,845 21.54 17.41 22

35-44 148,870 33.96 50.56 98

45-54 102,649 56.82 57.3 174

55-59 42,494 75.23 31.97 112

Totals 534,533 181.09 436

SMR = observed deaths for an occupation – cause – group / expected deaths for occupation – cause – group x 100SMR = 436/181.09x100 = 241

Compute SMR for Occupation A:

Age Standard rate Population from A Expected deaths

40-4950-59Total

Compute SMR for Occupation A:

Age Standard rate Population from A Expected deaths

40-4950-59Total

0.001 x0.003 x

1000 =5000 =

11516

SMR = (observed deaths) / (expected deaths) = 22 / 16 = 1.38

Compute SMR for Occupation B:

Age Standard rate Population from B Expected deaths

40-4950-59Total

Compute SMR for Occupation B:

Age Standard rate Population from B Expected deaths

40-4950-59Total

0.001 x0.003 x

5000 =1000 =

538

SMR = (observed deaths) / (expected deaths) = 14 / 8 = 1.75

Using SMR to compute age-adjusted rate

Indirectly standardized rate =SMR x [crude death rate in the standard pop]

Using SMR to compute age-adjusted rate

SMR = 1.38 1.75

IndirectlyStandardized 1.38 x 0.002 1.75 x 0.002Rate = = 0.003 = 0.0035

Crude death rate in standard pop = 150/70000 = 0.002 or 2 per 1000 pop


1. Direct

2. Indirect

http://www.epidemiolog.net/evolving/Standardization.pdf

•Use the populations from the reference (standard) population, and the rates from the test population

•Use the rates from the reference (standard) population, and the populations from the test population


Compute the age-adjusted mortality rates using direct standardization for A and B.

Compare the results to the indirect rate computed earlier.


Calculate SMRs and indirectly standardized age-adjusted rates for both pops.

HSS4303B – Intro to Epidemiology Jan 21, 2010 – Standardization.

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