Bio statistics 11
negative test). It is calculated mathematically by dividing the number of true negatives by the
total number of people with a negative test. NPV also depends on the prevalence of the disease
and the sensitivity and specificity of the test (the higher the prevalence, the lower the NPV). In
addition, an overly sensitive test with lots of false positives makes the NPV higher.
6. Define attributable risk. How is it measured?
Attributable risk is the number of cases of a disease attributable to one risk factor (in other
words, the amount by which the incidence of a condition is expected to decrease if the risk factor
in question is removed). For example, if the incidence rate of lung cancer is 1/100 in the general
population and 10/100 in smokers, the attributable risk of smoking in causing lung cancer is
9/100 (assuming a properly matched control).
7. You need to develop the habit of drawing a 2 X 2 table for Step 2 statistics questions.
Given the 2X2 table below, define the formulas for calculating the following test values:
Disease Test Name Formula
Sensitivity A/(A + C)
Specificity D/(B + D)
Test (+) A B PPV A/(A + B)
NPV D/(C + D)
Exposure (-) C D Odds ratio (A X D)/(B X C)
Relative risk [A/(A + B)]/[C/(C + D)]
Attributable risk [A/(A + B)]-[C/(C + D)]
8. Define relative risk. From what type of studies can it be calculated?
Relative risk compares the disease risk in people exposed to a certain factor with the disease
risk in people who have not been exposed to the factor in question. Relative risk can be calcu-
lated only after prospective or experimental studies; it cannot be calculated from retrospective
data. If a Step 2 question asks you to calculate the relative risk from retrospective data, the answer
is "cannot be calculated" or "none of the above."
9. What is a clinically significant value for relative risk?
Any value for relative risk other than 1 is clinically significant. For example, if the relative
risk is 1.5, a person is 1.5 times more likely to develop the condition if exposed to the factor in
question. If the relative risk is 0.5, the person is only half as likely to develop the condition when
exposed to the factor; in other words, the factor protects the person from developing the disease.
10. Define odds ratio. From what type of studies is it calculated?
Odds ratio attempts to estimate relative risk with retrospective studies (e.g., case control). An
odds ratio compares (the incidence of disease in persons exposed to the factor and the incidence
of nondisease in persons not exposed to the factor) with (the incidence of disease in persons unex-
posed to the factor and the incidence of nondisease in persons exposed to the factor) to see
whether there is a difference between the two. As with relative risk, values other than 1 are sig-
nificant. The odds ratio is a less than perfect way to estimate relative risk (which can be calcu-
lated only from prospective or experimental studies).
11. What do you need to know about standard deviation (SD) for the USMLE?
You need to know that with a normal or bell-shaped distribution, 1 SD holds 68% of the
values, 2 SD hold 95% of the values and 3 SD hold 99.7% of the values. A classic question gives
you the mean and standard deviation and asks you what percentage of values will be above a
given value. Variations in this question are common.
12. Define mean, median, and mode.
The mean is the average value, the median is the middle value, and the mode is the most
common value. A question may give you several numbers and ask you for their mean, median,
and mode. For example, if the question gives you the numbers 2, 2, 4, and 8:
