Statistical Correlations Review - Study Guide

What is a true positive versus a false positive?

• True positive (TP): Test = abnormal, gold standard = abnormal

• False positive (FP): Test = abnormal, gold standard = normal

What is a false negative versus a true negative?

• False negative (FN): Test = normal, gold standard = abnormal

• True negative (TN): Test = normal, gold standard = normal

What does “true” mean when classifying statistical test results?

The noninvasive test agrees with the gold standard. \

2 × 2 Table

In a 2 × 2 table, what do the rows and columns represent?

• Rows: Noninvasive test

• Columns: Gold standard

What do A, B, C, and D represent in the 2 × 2 table?

• A = TP

• B = FP

• C = FN

• D = TN

What does sensitivity measure, and what is the formula?

Sensitivity tells how well the test detects disease.

Sensitivity = TP ÷ (TP + FN)

Denominator = all gold-standard positives.

What does specificity measure, and what is the formula?

Specificity tells how well the test identifies normal patients.

Specificity = TN ÷ (FP + TN)

Denominator = all gold-standard negatives.

What does positive predictive value (PPV) measure, and what is the formula?

PPV tells the probability that a positive test is truly abnormal.

PPV = TP ÷ (TP + FP)

Denominator = all positive noninvasive tests.

What does negative predictive value (NPV) measure, and what is the formula?

NPV tells the probability that a negative test is truly normal.

NPV = TN ÷ (FN + TN)

Denominator = all negative noninvasive tests.

What does accuracy measure, and what is the formula?

Accuracy measures overall agreement/correct diagnoses.

Accuracy = (TP + TN) ÷ Total studies

If the duplex is normal but angiography is abnormal, what type of result is this?

False negative — the test missed the disease

If both the duplex and angiography are abnormal, what type of result is this?

True positive

Which metric answers “How well does the test detect disease?” and which metric uses all positive noninvasive tests as its denominator?

• Detect disease → Sensitivity

• All positive noninvasive tests → PPV

Which measure reflects overall correct diagnoses?

Accuracy

In Practice Case 1, what are the TP, FP, FN, and TN values?

• TP = 84

• FP = 16

• FN = 21

• TN = 79

What are the sensitivity and specificity in Practice Case 1?

• Sensitivity = 80.0%

• Specificity = 83.2%

What are the PPV, NPV, and accuracy in Practice Case 1?

• PPV = 84.0%

• NPV = 79.0%

• Accuracy = 81.5%

If a test has very high sensitivity but lower specificity, what does that mean?

It detects most patients who truly have disease

If a test result is negative, which metric tells the probability that it is truly normal?

Negative predictive value — NPV

Why should sensitivity and specificity be considered together?

Because one tells how well the test detects disease, while the other tells how well it identifies normal patients.

List the 3 key steps for solving statistical-correlation questions.

1. Classify the outcome correctly

2. Choose the denominator that matches the question

3. Calculate, round, and interpret the result