Section 3: 8 Measures of Frequency & Association

Question 1

What is a prevalence count?

Answer 1

number of cases at a particular time

Question 2

What is an incidence count?

Answer 2

number of case onsets that accumulate over time

Question 3

Why are ratios important in analysis of case counts?

Answer 3

Counts without context of population size are seldom useful

Question 4

What are 3 types of ratios used in epidemiology?

Answer 4

1. Rates - denominator contains element of time
2. Proportions - numerator is subset of denominator
3. Odds - numerator is not subset of denominator

Question 5

What are the two measures of disease frequency in a population?

Answer 5

prevalence and incidence

Question 6

What is prevalence?

Answer 6

proportion of population with condition

Question 7

What is incidence proportion (a.k.a. risk, cumulative incidence, expected proportion developing disease)?

Answer 7

number of new cases (onsets) divided by population at risk at a given time

Question 8

What is the relation between incidence

and prevalence?

Answer 8

prevalence ≅ (incidence rate)× (average duration)

P≅ ID

Question 9

What are two factors that may be reflected in a high incidence rate?

Answer 9

• • High risk

- - Prolonged survival without cure

Question 10

What are three factors that may be reflected in a low incidence rate?

Answer 10

• • Low risk
• • Rapid fatal disease progression
• • Rapid cure

Question 11

How do epidemiologists quantify the effect of E (exposure) on D (disease)?

Answer 11

comparing rates of disease in an exposed group (E+) and non-exposed groups (E-)

Question 12

What are the 5 steps of epidemiological reasoning?

Answer 12

1. Suspicion that a factor (exposure) may influence occurrence of disease
2. Formulation of specific hypotheses
3. Conduct analytic studies
4. Assess validity of association
5. Make a judgment of whether a cause-effect relation between factor (exposure) and disease exist

Question 13

What is a risk factor?

Answer 13

A factor (exposure) found to be associated with a health condition and that increases the probability of occurrence of disease

Question 14

What are some examples of risk factors?

Answer 14

• behavior
• genetic
• environmental
• social

Question 15

What is epidemiologic association?

Answer 15

• • Statistical relationship between two or more events, characteristics, or other variables
• • Statistical relationship between exposure and disease

Question 16

What are the two types of epidemiologic measures of

association?

Answer 16

absolute and relative

Question 17

What is Risk Difference and how is it calculated?

Answer 17

the absolute elevation in risk associated with an exposure.
RD = R1 − R0
Where R1 represents the risk/rate in the exposed group
R0 represents the risk/rate in the non-exposed group

Question 18

What is Relative Risk and how is it calculated?

Answer 18

the relative risk associated with an exposure:
RR=R1/R0
Where R1 represents the risk/rate in the exposed group
R0 represents the risk/rate in the non-exposed group

Question 19

How is the value of the relative risk calculation interpreted to determine if there is an association between exposure and disease?

Answer 19

1 = No association between exposure & disease (incidence rates are identical between groups)
>1 = Positive association (exposed group has higher incidence than nonexposed group)
<1 = Negative association or protective effect (non-exposed group has higher incidence)

Question 20

How are 2x2 tables used in epidemiology?

Answer 20

to summarize frequencies of disease and exposure and used for calculation of association

Question 21

How are odds ratios used in epidemiology?

Answer 21

The ratio of the odds of a condition in the exposed compared with the odds of the condition in the unexposed. Usually used in prevalence studies rather than incidence studies.

Question 22

What is attributable risk?

Answer 22

# of cases among the exposed that could be eliminated if the exposure were removed
AR = Incidence in exposed - Incidence in unexposed

Question 23

What is population attributable risk percent?

Answer 23

Proportion of disease in the study population that
could be eliminated if exposure were removed.
PAR = (Incidence in total population) - (Incidence in unexposed) divided by (Incidence in total population)

Question 24

What are the two types of differences measures? (difference that removal of an exposure would make)

Answer 24

Attributable Risk (AR) and Population Attributable Risk (PAR)

Question 25

What forms the basis for suspicions that a factor (exposure) may influence occurrence of disease?

Answer 25

• • Observations in clinical practice
• • Examination of disease patterns
• -Observations in laboratory research
• • Theoretical speculation

Question 26

What do epidemiologists look at when assessing the validity of an observed association?

Answer 26

whether there are alternative explanations for the

association (e.g., chance, bias, confounding)

Question 27

What are measures of association?

Answer 27

Calculations used to measure disease frequency

relative to other factors; indications of how more or less likely one is to develop disease as compared to another

Question 28

How are odds calculated?

Answer 28

p= p/1-p
Where p = the probability of an event
and 1-p = the probability that the event does not occur