Lecture 5 - Classification, diagnoses, and screening

Question 1

What is the advantage of using regression methods for determining whether a change in scores represents clinically significant change?

Answer 1

Regression methods can take into account practice effects

(and other factors, like the lapsed time between tests)

Question 2

How would you describe the concept of test SENSITIVITY, with reference to test results and the proportion of cases who have the target condition?

Answer 2

SENSITIVITY = the proportion of cases who HAVE the target condition for whom the test result is POSITIVE

Question 3

How would you describe the concept of test SPECIFICITY, with reference to test results and the proportion of cases who have the target condition?

Answer 3

SPECIFICITY = the proportion of cases who DON’T HAVE the target condition for whom the test result is NEGATIVE

Question 4

What simple equation is used to determine SENSITIVITY, using values from the “confusion matrix”?

Answer 4

SENSITIVITY = True positives / (True positives + False negatives)

Put differently, it means:

True positives divided by everyone who HAS the condition

Question 5

What simple equation is used to determine the SPECIFICITY, using values from the “confusion matrix”?

Answer 5

SPECIFICITY = True negatives / (True negatives + False positive)

Put differently, it means:

True negatives, divided by everyone who DOESN’T have the condition

Question 6

What is the counter-intuitive name for the “curve” that is used to determine optimal sensitivity and specificity?

Answer 6

The receiver operating characteristic (ROC) curve

Question 7

What THREE pieces of information are required to determine the PPV and NPV?

Answer 7

1. Sensitivity (of the test)
2. Specificity (of the test)
3. Base rates (in a relevant population)

Question 8

What is the equation for calculating the LR+?

Answer 8

LR+ = Se / (1- Sp)

Where Se is the test sensitivity and Sp is the test specificity

Question 9

What is the equation for calculating the LR-?

And what is the extra step that is sometimes applied to make it more intuitive?

Answer 9

LR- = (1 - Se) / Sp

Where Se is the test sensitivity and Sp is the test specificity

The resulting number is then converted to a more intuitive figure using the following calculation

1/ LR