U2 L2 Statistical Correlations

What are statistics, and what does Quality Assurance determine?

• Statistics: Gathering facts as numbers and presenting them to show information

• Quality Assurance: Determines the quality of the diagnostic test being considered

What are QA statistics, and what is the gold standard?

QA statistics evaluate the value of a noninvasive study by comparing it with a reference test assumed to be the truth.

The reference test is the gold standard

What do statistics tell us about a noninvasive study?

• Provide a level of confidence in the results

• Identify limitations of the study, sonographer, and/or reading physician

Ideally, how closely should a noninvasive test match the gold standard, and what correlation is realistically considered good?

• Ideally: 100%

• Realistically: 90% correlation is good

What is the gold standard listed in this PowerPoint?

Angiography

What is a true positive versus a false positive?

• True positive: Vascular lab and gold standard both agree the test is positive

• False positive: Vascular lab says abnormal, but gold standard says normal

What is a true negative versus a false negative?

• True negative: Vascular lab and gold standard both agree the test is negative

• False negative: Vascular lab says normal, but gold standard says abnormal

List the 5 measures of statistics.

• Sensitivity

• Specificity

• Positive predictive value

• Negative predictive value

• Accuracy

Which 2 measures should both be considered when evaluating the benefit of an exam?

• Sensitivity

• Specificity

What is sensitivity?

The ability to detect disease.

In simple terms: How well can the test detect abnormal/diseased patients?

What is specificity?

The ability to identify normality.

In simple terms: How well can the test identify normal patients?

What is positive predictive value (PPV)?

The percentage of positive noninvasive test results that correctly predict disease/abnormality when compared with the gold standard

What is negative predictive value (NPV)?

The percentage of negative noninvasive test results that correctly predict the absence of disease/normality when compared with the gold standard.

What is accuracy?

The percentage of correct noninvasive diagnoses

It tells how well the test can both detect and rule out disease

What happens to accuracy when more vascular-lab results agree with the gold standard?

Accuracy increases.

What is the purpose of the 2 × 2 factorial table?

It is a well-accepted method of calculating statistics that compares the noninvasive test with the gold standard.

In the 2 × 2 factorial table, what do + and − mean?

• + = abnormal exam

• − = normal exam

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

• A = True positive

• B = False positive

• C = False negative

• D = True negative

What is the sensitivity formula?

Sensitivity = A ÷ (A + C)

or

True positives ÷ all positive diagnoses by the gold standard

What is the specificity formula?

Specificity = D ÷ (B + D)

or

True negatives ÷ all negative diagnoses by the gold standard

What is the PPV formula?

PPV = A ÷ (A + B)

or

True positives ÷ all positive noninvasive diagnoses

What is the NPV formula?

NPV = D ÷ (C + D)

or

True negatives ÷ all negative noninvasive diagnoses

What is the accuracy formula?

Accuracy = (A + D) ÷ (A + B + C + D)

or

Total correct tests ÷ total number of studies

In the first example, what are the TP, FP, FN, and TN values?

• TP = 185

• FP = 20

• FN = 10

• TN = 35

What are the sensitivity, specificity, PPV, NPV, and accuracy in the first example?

• Sensitivity = 94.9%

• Specificity = 64%

• PPV = 90.2%

• NPV = 77.8%

• Accuracy = 88%

In the second example, what are the TP, FP, FN, and TN values?

• TP = 75

• FP = 20

• FN = 10

• TN = 70

What are the sensitivity, specificity, PPV, NPV, and accuracy in the second example?

• Sensitivity = 88.2%

• Specificity = 77.7%

• PPV = 79%

• NPV = 87.5%

• Accuracy = 83%