EPIDEMIOLOGY/BIOSTATISTICS

Question 1

Number needed to treat

Answer 1

Number of pts who need to be treated for 1 patient to benefit.

=1/absolute risk reduction

Question 2

Positive predictive value (PPV)

Answer 2

= TP / (TP + FP)

Proportion of positive test results that are true
positive.

Probability that person actually has the disease
given a positive test result

PPV varies directly with prevalence or pretest
probability: high pretest probability p high PPV

Question 3

Relative risk (RR)

Answer 3

Risk of developing disease in the exposed group
divided by risk in the unexposed group

(e.g., if 21% of smokers develop lung cancer vs. 1% of
nonsmokers, RR = 21/1 = 21).

If prevalence is low, RR ≈ OR.

RR=
a/(a + b)
————
c/(c + d)

Question 4

Specificity (truenegative rate)

Answer 4

= TN / (TN + FP)

= 1 – false-positive rate

Question 5

Both the _________ and ______________ vary widely with the prevalence of a disease within a population.

Answer 5

positive predictive value and the negative predictive value

Question 6

Sensitivity (true positive rate) equation

Answer 6

= TP / (TP + FN)

    \+       -
----------------
\+ | TP | FP
----------------
-  | FN | TN

Question 7

Specificity (true negative rate) equation

Answer 7

= TN / (TN + FP)

    \+       -
----------------
\+ | TP | FP
----------------
-  | FN | TN