Lecture 3

Question 1

What types of study designs use prevalence?

Answer 1

cross-sectional, some cohort, and case-control studies

Question 2

What types of study designs use incidence?

Answer 2

most cohort and randomized controlled trials (RCTs)

Question 3

What can incidence be calculated as and what must each have?

Answer 3

can be calculated as a proportion (defined time interval) or a rate (person-time denominator)

Question 4

What is the incidence rate (IR) equation?

Answer 4

IR = # cases / person-time total of ALL participants

Question 5

What is person-time?

Answer 5

the number of years that a participant contributed to a study before developing the disease, dropping out, or passing away

Question 6

What is cumulative incidence (CI)?

Answer 6

it is the risk reflecting the proportion of participants who developed the disease over a period of time as a % (needs ALL initial participants to follow up/complete study)

Question 7

What is the CI equation?

Answer 7

CI = # persons who developed the disease during a period of time / # persons at risk at BASELINE who stay till the end of the study and remain disease free (only those with risk factor or disease free at the time of calculation)

Question 8

What are the difference measures that can be ran on incidence values?

Answer 8

• risk difference (RD) or absolute risk reduction (ARR)
• number needed to treat (NNT)
• risk ratio or relative risk (RR)

Question 9

What is the RD equation?

Answer 9

RD = IR or CI exposed - IR or CI unexposed
- all values are absolute

Question 10

What would the interpretation of the RD value be?

Answer 10

RD is positive- risk for exposed > unexposed
RD is negative- risk for exposed < unexposed

Question 11

What is the number needed to treat equation?

Answer 11

NNT = 1 / RD (always round value up)

Question 12

What would the interpretation of the NNT value be?

Answer 12

the value is the number of individuals that would need to be treated to prevent 1 case of the disease/outcome

Question 13

What is the relative risk (RR) equation?

Answer 13

RR = CI exposed / CI unexposed (always controls)
(never negative value)

Question 14

What would the interpretation of the RR value be?

Answer 14

value > 1- risk is HIGHER in the numerator (exposed) than the denominator (unexposed)
value < 1- risk is LOWER in exposed than unexposed

Question 15

What is the prevalence point (PP) equation?

Answer 15

PP = # persons with disease / # persons examined at BASELINE of the group being examined (cases and controls)

Question 16

What are the difference measures that can be ran on prevalence value?

Answer 16

• prevalence difference (PD)
• population attributed risk (PAR)
• odds ratio (OR)

Question 17

What is the prevalence difference (PD) equation?

Answer 17

PD = PP exposed - PP unexposed

Question 18

What would the interpretation of the PD value be?

Answer 18

PD is positive- risk for exposed > unexposed
PD is negative- risk for exposed < unexposed

Question 19

What is the population attributed risk (PAR) equation?

Answer 19

PAR = [PP overall - PP unexposed] / PP overall
- represented as a %

Question 20

What would the interpretation of the PAR value be?

Answer 20

the % value is the % of prevalent cases that are correlated the the exposure that could be eliminated if the exposure were to be removed

Question 21

What is the odds ratio (OR) equation?

Answer 21

OR = [PP exposed / (1 - PP exposed)] / [PP unexposed / (1 - PP unexposed)]

Question 22

What would the interpretation of the OR value be?

Answer 22

OR < 1- having the exposure LOWERS the odds of obtaining the outcome (has decreased risk)
OR > 1- having the exposure INCREASES the odds of obtaining the outcome (has increased risk)

Question 23

What type of study design can RR not be estimated for?

Answer 23

case-control studies