Epi Class 5

Question 1

Interpretation of the Rate Ratio (RR):

Answer 1

The RR is the number of times greater the risk of disease in the exposed compared to the unexposed.

Relative Rate (RR) says nothing about absolute risk. It is the rate of incident disease in the exposed compared to the rate of incident disease in the unexposed. (It is possible for a disease with a very high RR to still have very low incidence, even in the exposed group.)

Question 2

RR formula

Answer 2

RR = a / (a + b)
————————-
c / (c + d)

Question 3

Key measure for Cohort Studies

Answer 3

RR (rate ratio) = IRR (incidence rate ratio)

Question 4

Key measures for Case-Control Studies

Answer 4

OR (odds ratio)

Question 5

What do Cohort Studies evaluate?

Answer 5

Cohort Study: Measure the incidence of disease in the exposed versus non-exposed populations.

Question 6

What do Case-Control Studies evaluate?

Answer 6

Case-Control Study: Compare the exposure history in people with the disease (cases) and people without the disease (controls).

Question 7

Three types of RR associations

Answer 7

Positive Association= RISKY EFFECT

No Association= NULL ASSOCIATION

Negative Association= PROTECTIVE EFFECT

Question 8

Attributable Risk (Risk Difference, AR)

Answer 8

the excess rate of disease in the exposed – that is, the incidence of disease
among the exposed that is due to the exposure.

Question 9

Attributable Risk Formula

Answer 9

AR = Incidence in exposed – Incidence in unexposed

= IE – IU = a - c
———- ———-
a + b) (c + d

Question 10

Definition of Attributable Risk Percent (Etiologic Fraction, AR%)

Answer 10

the proportion of disease among the exposed that is due to the exposure.

Question 11

Attributable Risk Percent (%) Forumla

Answer 11

IE – IU
AR% = ————– x 100
IE

or
RR – 1
AR% = ————– x 100
RR

Question 12

Population Attributable Risk (PAR)

Answer 12

the excess rate of disease in the total population that is due to the exposure.

Question 13

Population Attributable Risk (PAR) Formula

Answer 13

PAR = It – Iu = AR x Pe

To calculate the PAR you must know the prevalence of exposure in the population (Pe). This can be a previously reported Pe from a population survey or the Pe calculated from your cohort study.

Question 14

Prevalence of Exposure (Pe) Formula

Answer 14

Pe = (a + b) ÷ (a + b + c + d)

Question 15

Incidence in the total population (It) formula

Answer 15

It = (a + c) ÷ (a + b + c + d)

Question 16

It

Answer 16

Incidence in the total population

Question 17

Iu

Answer 17

Incidence in the unexposed population

Question 18

Population Attributable Risk Percent (PAR%) Formula

Answer 18

PAR
PAR% = ——- x 100
It

Question 19

Odds Ratio (OR)

Answer 19

Measure of association. Used in Case-Control studies.

Question 20

Odds Ratio (OR) Formula

Answer 20

OR = a / c ÷ b / d = ad / bc

Odds of exposure among the non-diseased (controls)

Question 21

appropriate measure of association

Answer 21

Odds Ratio

Question 22

appropriate measure of incidence

Answer 22

Rate Ratio

Question 23

Interpretation of the Odds Ratio (OR)

Answer 23

The OR is the number of times greater the odds of exposure in the disease compared to the not diseased.

POSITIVE association = implies RISKY EFFECT

NO association = NULL association

NEGATIVE association = implies a PROTECTIVE EFFECT

Question 24

95% Confidence Intervals (CI)

Answer 24

95% CIs are required for determining statistical significance.

If the whole bar is under “1” then exposure is PROTECTIVE

If the whole bar is over “1” then exposure is RISKY

If the bar is on both sides of “1” then exposure has NO RISK ASSOCIATION

Question 25

Types of matching

Answer 25

Frequency matching (group matching): case group and control group have similar profiles (% women, % by age, etc.)
     - no need for special OR: use normal OR

Individual matching (matched-pairs matching): each case is matched to one or more individuals who is very similar (most often used in GENETIC studies where biologically-related family members are linked)
     - must use matched-pairs OR

Question 26

Matched-pairs Analysis

Answer 26

REVIEW TABLE IN NOTES!

For individual matching, a special kind of 2x2 table is used. Look for:
Concordant pairs (a and d): same exposure experience for case and control

Discordant pairs (b and c): different exposure experience for cases and controls

Matched pairs “OR” (not really an odds ratio) = ratio of discordant pairs = b/c

Question 27

Discordant Pairs Odds Ratio

Answer 27

b / c