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

2. Survival

3. 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

5. 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%

6. 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.

7. 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 2 years) /
(% of general population who are 2 yrs)
Why is this important?
Eg, The overall 5-year relative survival rate for 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.

8. 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.
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.
The 5-year cause-specific survival for stage IIA Hodgkin lymphoma is 85% when treated with ABVD followed by involved field radiation.

9. 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

11. Cause of death from a death certificate

12. 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

13. Changes in death rates from diabetes caused by changes in classification

14. 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

15. AIDS cases in the US from 1984-2000

16. 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"

17. Standardization

18. 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?

19. What is this an example of?
Con…..

20. So What Can We Do To Eliminate the Uncertainty?

21. 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

22. 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

23. 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

24. Two Kinds of Age Standardization
Direct
Indirect
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.

25. Two Kinds of Age Standardization
Direct
Indirect
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.

26. There are three major components that are needed to perform adjusted mortality rate calculations:
the number of deaths
the population
a "standard" population

27. Direct Standardization

28. Raywatville
Gomesland
Reference Pop.

29. 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

30. Directly Standardized Rate for Raywatville (Miami) is…

31. Directly Standardized Rate for Gomesland (Alaska) is…

32. Compare Age-Standardized Rates
Before age-adjustment:
Raywatville = 8.92 deaths/ 1000 people
Gomesland = 2.67 deaths/ 1000 people ?
After age-adjustment:
Raywatville = 6.92 deaths/ 1000 people
Gomesland = 6.71 deaths/ 1000 people

33. 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

34. 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?

35. Gomesland
Raywatville

36. Age group
Raywatville population (weight)
Gomesland age-specific rate
Gomesland age-adjusted deaths
<15
15-24
25-44
45-64
65+
114350
80259
133440
142670
92168
1.59
0.9
1.13
6.02 39

37. Age group
Raywatville population (weight)
Gomesland age-specific rate
Gomesland age-adjusted deaths
<15
15-24
25-44
45-64
65+ x x x x x = = = = =
TOTALS
562887
Directly standardized rate = / = 8.63 deaths / 1000 pop

38. Compare Age-Standardized Rates
Before age-adjustment:
Raywatville = 8.92 deaths/ 1000 people
Gomesland = 2.67 deaths/ 1000 people ?
After age-adjustment:
Raywatville = 8.92 deaths/ 1000 people
Gomesland = 8.63 deaths/ 1000 people

39. Your Homework Assignment #1
Compute Age-Adjusted Rates in the previous example, using Gomesland as the standard population, rather than Raywatville
Then compute Age-Adjusted Rates using the hybrid population of “Gomesland + Raywatville” as the standard population
How do your conclusions differ?

40. Homework Assignment #2
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.
Early Period
Later Period
Population
No. of Deaths
Death Rate per 100,000
900,000
862
1,130
Early Period
Later Period
Age Group (yr)
Population
No. of Deaths
Death Rates per 100,000
All ages
900,000
862 96
1,130
126
30-49
500,000 60
300,000 30
50-69
396
400,000
400
70+
100,000
406
200,000
700
350

41. Homework Assignment #3
Fill in the table and compue adge-adjusted rates for both Mexico and the USA, using the given 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
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

42. Indirect Age Standardization
Involves the computation of a Standardized Mortality Ratios (SMR)
SMR= observed number of deaths per year
expected 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

43. 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

44. SMR for tuberculosis
Table 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 100
SMR = 436/181.09x100 = 241

45. SMR for tuberculosis
Table 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 100
SMR = 436/181.09x100 = 241

46. Compute SMR for Occupation A:
Age
Standard rate
Population from A
Expected deaths
40-49
50-59
Total

47. Compute SMR for Occupation A:
Age
Standard rate
Population from A
Expected deaths
40-49
50-59
Total x x = = 1 15 16
SMR = (observed deaths) / (expected deaths)
= 22 / 16
= 1.38

48. Compute SMR for Occupation B:
Age
Standard rate
Population from B
Expected deaths
40-49
50-59
Total

49. Compute SMR for Occupation B:
Age
Standard rate
Population from B
Expected deaths
40-49
50-59
Total x x = = 5 3 8
SMR = (observed deaths) / (expected deaths)
= 14 / 8
= 1.75

50. Using SMR to compute age-adjusted rate
Indirectly standardized rate = SMR x [crude death rate in the standard pop]

51. Using SMR to compute age-adjusted rate
Crude death rate in standard pop = 150/70000 = or 2 per 1000 pop
SMR =
Indirectly
Standardized x x 0.002
Rate = = =

52. Two Kinds of Age Standardization
Direct
Indirect
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

53. Homework Assignment #4
Compute the age-adjusted mortality rates using direct standardization for A and B.
Compare the results to the indirect rate computed earlier.

54. Homework Assignment #5
Calculate SMRs and indirectly standardized age-adjusted rates for both pops.