Epidemiology | Biostatistics

Question 1

Likelihood ratios
a) Used to assess what?
b) Definitions (positive vs negative)
c) How to calculate

Answer 1

a) Assesses 1. potential utility of a particular diagnostic test. 2. how likely it is that a patient has a disease or condition.

b) Positive likelihood ratio (LR+) = probability that a person with the disease tested positive for the disease (true positive) divided by the probability that a person without the disease tested positive for the disease (false positive).

Negative likelihood ratio (LR-) =
probability that a person with the disease tested negative for the disease (false negative) divided by the probability that a person without the disease tested negative for the disease (true negative).

c)
LR+ = sensitivity / 1 – specificity
LR- = 1 – sensitivity/specificity

Question 2

Define effect size

Answer 2

Magnitude of the difference between groups. The absolute effect size is the difference between the mean outcomes in 2 different groups.

Question 3

Define hazard ratio

Answer 3

A measure of how often a particular event happens in one group compared to how often it happens in another group, over time.
HR = 1 means lack of association, HR >1 suggests an increased risk, HR <1 suggests a smaller risk.
E.g. HR of 0.5 means that half as many patients in the group have an event at any point in time vs placebo proportionately.

Question 4

Difference between predictive value & likelihood ratio?

Answer 4

PVs are affected by disease prevalence, but LRs are not.