Measures of Association

Intermediate Epidemiology

Objectives

• To play “name that rate”

• To discuss case-control study designs

• To discuss and calculate measures of association

• To discuss the design and measures of association of a study

Name that Rate

• What’s the numerator

• What’s the denominator

• What is the type of rate

0.2

3.2

0.9

0.1

N=0N=2

N=84N=237N=58

AmericanIndian/

Alaska Native

WhiteNot Hispanic

BlackNot Hispanic

Hispanic Asian/PacificIslander

0

1

2

3

4

5

AIDS Rates per 100,000 Children <13 Years of Ageby Race/Ethnicity, Reported in 1998*, United States

Race/Ethnicity*US Rate=0.7/100,000 N=382

Rate

per

10

0,0

00

4.7

13.5

VI 29.6PR 44.3

<55 - 14.915+

AIDS Rates per100,000 PopulationReported in1998

Rate per 100,000

DE

MARICTNJ

MDDC

NH

26.320.3

23.4

189.131.9

15.012.9

3.5

11.1

13.88.0

7.9

36.5

16.9

2.6

10.8

2.6

4.87.1

21.8

2.5

7.3

4.0

15.1

8.1

3.3

4.314.8

12.0

47.9

10.4

0.9

6.18.2

8.5

6.2

14.5

20.3

2.0

12.8

20.1

6.6

14.7

7.8

4.7

3.9

17.3

1.2

VT3.4

0

10

20

30

40

50

60

70

Year of Report

American Indian/Alaska Native

Black, not Hispanic

Hispanic

Asian/Pacific Islander

White, not Hispanic

1985 1987 1989 1991 1993 1995 1997

Proportion of AIDS Cases, by Race/Ethnicity and Year of Report,1985-1998, United States

Perc

en

t of

Case

s

AIDS Cases in Adult/Adolescent Women by Race/Ethnicity per 100,000 Population, Reported in 1998,

United States

Race/Ethnicity

White, not Hispanic

Black, not Hispanic

Hispanic

Asian/Pacific Islander

American Indian/Alaska NativeTotal*

Cases

2,031

6,775

2,055

59

3010,998

Rate

2

50

17

1

410

*Includes 48 women whose race/ethnicity is unknown.

Born1993-March 1998 in 29 States‡, United Stateswho Received or whose Mothers Received any ZDV*

0

20

40

60

80

100

* Any ZDV=Prenatal, intrapartum, or neonatal receipt of Zidovudine to reduce perinatal HIV transmission

Quarter-Year of Birth

1993N=1313

1994N=1271

1995N=1315

1996N=1260

1997N=1291

1998N=281

‡ Includes 29 areas that have conducted pediatric HIV Surveillance since 1993; data reported through March 1999

Percent of Perinatally HIV Exposed or Infected ChildrenPerc

ent

Rece

ivin

g Z

idovudin

e

NortheastN=14,399

North CentralN=4,317

South N=19,474

WestN=8,121

Metropolitanarea 50,000-

499,999 population

Non-metropolitanarea

AIDS Cases by Region and Size of Place of ResidenceReported in 1998, United States

%

93.1 4.5 2.4

76.3 14.4 9.3

75.6 13.5 10.9

86.7 8.2 5.0

% %

Metropolitanarea 500,000population

71%

12%

13%

AIDS Cases N=48,269*

PopulationN=274,766,000

AIDS Cases Reported in 1998 and Estimated 1998 Population, by Race/Ethnicity, United States

White, not HispanicBlack, not Hispanic

Hispanic

Asian/Pacific IslanderAmerican Indian/ Alaska Native

33%

45%20%

*Includes 211 persons with unknown race/ethnicity

<1%1%

1%4%

Exposure Category N=15020-2913-19

N=2,01530-49N=7,683

50+N=1,150

Injection drug use 9

% % % %

19 34 19

Heterosexual contact 41 46 35 39

Transfusion recipient 2 1 1 3

Other/not identified* 48 34 30 39

Age at Diagnosis (in years)

*Includes patients whose medical record review is pending; patients who died, were lost to follow-up, or declined interview; and patients with other or undetermined modes of exposure

AIDS in Women, by Exposure Category andAge at Diagnosis, Reported in 1998, United States

Case-control study

• Persons are categorized by disease status and then compared for their exposure status

• The odds ratio is the measure of association (an estimate of relative risk)

• The assumption is that the cases and controls originate from the same hypothetical source cohort.

• When this assumption is not met, the results are susceptible to selection bias

• Traditional method is case-based case-control study

Nested case-control or case-cohort

• When cases are identified within a well defined cohort

• Also known as a hybrid or ambi-directional study

Alternatives for selection of controls and a case-control study

• Case-cohort - When the controls are randomly selected from a defined cohort at baseline.

• Nested case-control – based on incidence density sampling (matching cases and controls on duration of follow-up) or risk-set sampling (or comparison of cases with a subset of the cohort members as risk of being cases at the time when each case occurs)

Case-crossover

• Compares the exposure status of a case immediately before it’s occurrence with that of the same case at some other prior time

• Appropriate for the study of acute (brief) exposures that produce transient change in risk of an acute condition (eg, MI and episodes of anger or sexual activity)

Matching

• Performed to reduce confounding

• Can be individually matched or frequency matched

Advantages of Matching

• May be the only way to control some degrees of confounding (especially very strong confounding)

• Tends to increase the statistical power

• Logistically straight forward

Disadvantages of Matching• May be impossible to find a control• When matching is done, the association between the

matched variables and the outcome can no longer be assessed

• Also eliminates the ability to assess additive interaction between the matched variable and the exposure of interest

• Special statistical tests need to be used• By matching you remove the representativeness of your

controls• Once matching is done, it cannot be undone• No statistical power is gained if the matching variables are

weak confounders

Relative Risk for a disease exposure

CVD No CVDObesity 75 25 100No Obesity 25 75 100

100 100 200

RR = 75/100 = 3.00 25/100

C.I. (2.10 - 4.29)

Relative Risk for preventive intervention

Disease No DiseaseCounseling 25 75 100No Counseling 50 50 100

75 125 200

RR = 25/100 = .50 50/100

C.I. (.39-.79)