Biostats

Question 1

What is effect modification?

Answer 1

Results when an external variable (effect modifier) has a positive or negative impact on the observed effect of a risk factor (exposure) on disease status (outcome). For instance, aspirin use is associated with Reye syndrome in children but not adults; therefore, age (effect modifier) modifies the effect of aspirin (exposure) on Reye syndrome development (outcome).

Question 2

How are the positive and negative likelihood ratios calculated?

Answer 2

LR+ = sensitivity / (1 - specificity).  This is the probability of a patient with the disease testing positive divided by the probability of a patient without the disease testing positive.  It is equivalent to: (true positive rate/false positive rate).
LR- = (1 - sensitivity) / specificity.  This is the probability of a patient with the disease testing negative divided by the probability of a patient without the disease testing negative.  It is equivalent to: (false negative rate / true negative rate).