Risk

Question 1

Define prevalence probability?
Define incidence probability?

Answer 1

Prevalence probability- probability HAVING disease at given point in time
Incidence probability- probability GETTING disease during specified point in time

Question 2

Define incidence rate?
Define hazard rate?

Answer 2

Incidence rate- Avg rate of change over time
Hazard rate- Instantaneous rate of change

Question 3

State 2 ways to calculate probability?

Answer 3

p = no. favourable outcomes/all possible outcomes
p = no. of cases/total population

Question 4

Define conditional probability?
How written?

Answer 4

Probability that something will happen given an event has already happened
E.g a patient will test positive IF they already have disease
Written P(A|B)

Question 5

Define odds?
Roll fair dice

Answer 5

Cases/noncases
Eg. roll fair dice= 1:5=0.2

Question 6

What does probability range from?
What does odds range from?

Answer 6

Prob- 0-1
Odds- 0 to infinity
- 1 indicate positive association
- 1 indicate no association

Question 7

What does odds ration of 1 indicate?
What does odds ration of more than 1 indicate?

Answer 7

Odd ratio 1- no association
Odd ratio 1+- positive association

Question 8

State issues using prevalence probability?

Answer 8

Length-time bias- conditions longer duration more likely captured
Diff in prev. can simply indicate shorter/longer duration disease or diff treatment success

Question 9

Issues with incidence probability?

Answer 9

Issues competing risk- death due another cause
Ignore time to event

Question 10

State risk difference and risk ratio:
Drug A- 0.6
Drug B- 0.2

Answer 10

Risk difference- 0.4 (40%)
Risk ratio- 3 (3x risk)

Question 11

State absolute risk difference and relative risk difference:
Drug A- 0.6
Drug B- 0.2

Answer 11

Absolute risk difference- A-B= 0.4
Relative risk difference- (A-B/B)*100= 200%

Question 12

Issues with relative risk?

Answer 12

Exaggerate risk
Don’t indicate baseline

Risk diff and ration give clearer sense impact/effect

Question 13

Define relative risk?

Answer 13

ratio of the probability of an event occurring with an exposure versus the probability of the event occurring without the exposure.

Question 14

How calculate relative risk- how much more likely?

Answer 14

Question 15

Calculate relative risk?

Answer 15

Question 16

Explain result:
RR of 1
RR greater 1
RR less 1

Further from 1 means?

Answer 16

RR=1 no association, no increased risk
RR greater 1= incidence exposed greater incidence non exposed- increased risk- positive association
RR less 1= incidence in exposed lower incidence in non exposed- decreased risk- negative association

Further from 1- stronger association

Question 17

What makes something statistically significant?

Answer 17

p value less 0.05
If ci doesn’t include 1

Question 18

When can’t calculate incidence in case control?
What use

Answer 18

50/50 way study designed
Use odd ratio

Question 19

State equation odds?

Answer 19

Odds= Probability/(1-Probability)
Eg. 60% prob win race
Odds 1/5 (60:40)

Question 20

Define RR
Define OR
with equation

Answer 20

Question 21

State how calculate odds ration?

Answer 21

Question 22

Calculate odd ratio

Answer 22

Question 23

When to use odd ration?

Answer 23

Cohort or case control

Question 24

How calculate odd ration in cohort?
How calculate odd ratio in case control?

Answer 24

Question 25

Issue odds ratio?

Answer 25

Overestimate risk

Question 26

Explain number needed to treat or harm?
Equation
If RD positive?

Answer 26

It is the number of people you need to treat in order to prevent one outcome
Indicate potential benefit of clinical intervention
NNT = 1/|RD|
RD is positive the intervention is harmful

Question 27

RD for MI = 0.03-0.05 = -0.02
NNT = 1/|RD| = 1/0.02 = 50

Drug A prevents MIs but also increases risk of lung cancer:
RD = 0.02-0.01
NNH = 1/RD = 1/0.01 = 100

Meaning?

Answer 27

Need treat 50 with drug A prevent 1 heart attack
Need to treat 100 people with drug A to cause 1 lung cancer

Question 28

Calculate NNH?

Answer 28

RD for MI = 0.03-0.05 = -0.02
NNT = 1/|RD| = 1/0.02 = 50

Drug A prevents MIs but also increases risk of lung cancer:
RD = 0.02-0.01
NNH = 1/RD = 1/0.01 = 100

Need treat 50 with drug A prevent 1 heart attack
Need to treat 100 people with drug A to cause 1 lung cancer