STATISTICAL EVALUATION OF LAB TESTS
TP = True Positive
FP = False Positive
TN = True Negative
FN = False Negative
|Sensitivity =||__ TP __||x 100 =||The frequency of positive test results among all the true positives. A test with high sensitivity will have a low number of false-negatives; but possibly a high number of false-positives.|
|TP + FN|
|Specificity =||__ TN __||x 100 =||The frequency of negative test results among all the true negatives. A test with high specificity will have a low number of false-positives; but possibly a high number of false-negatives.|
|FP + TN|
|Positive Predictive Value =||__ TP__||x 100 =||The probability that a positive test result will be correct.|
|TP + FP|
|Negative Predictive Value =||__ TN __||x 100 =||The probability that a negative test result will be correct.|
|TN + FN|
|Test Efficiency =||____ TP + TN ____||x 100 =||Specifies a test's overall accuracy. The probability that a test result, either positive or negative, correctly predicts the presence or absence of disease.|
|total number of tests|
Glossary of Statistical Terms
Accuracy = Describes the closeness of a test result to the actual value.
Analysis of Variance (ANOVA) = Estimates total standard deviation of control results analyzed over many days.
Bias = Difference between two quantities; "mean difference for all pairs of results".
Coefficient of Variation = The standard deviation divided by the mean.
Correlation Coefficient (r) = Assesses the reliability of regression estimates of slope and intercept; "describes the ellipse that encloses the results".
Imprecision = Coefficient of variation of the results in a replication experiment. (3)
Inaccuracy = Disagreement between an estimate and its true value. (3)
Line of Identity = Perfect relationship between a test method and reference method.
Mean = Arithmetic average.
Medical Decision Level (Xc) = Concentration where medical interpretation is critical for patient care.
Precision = Describes the reproducibility or consistency of a series of test results.
Reliability = Describes the degree of accuracy and precision of a test procedure.
Mean = The arithmetic "average". The sum of all values divided by the number of values.
Median = The middle value of a group of values i.e., halfway between the lowest and highest value.
Random Error (RE) = "difference between results due to random effects;" unpredictable.
Range = The difference between the highest and lowest value.
Regression Line (best fit) = Reduces the effect of RE and allows calculations of bias; estimates the meter value.
Sample Size (n) = total number of observations.
Slope (b) = Proportional relationship between two sets of results.
Standard Deviation = A measure of the dispersion of a set of data around the mean. An estimate of the degree of uniformity of the data.
Standard Error of the Estimate (Sy/x) = Measures the dispersion around the regression line.
Systematic Error (SE) = Disagreement between methods; error in one direction; consistently high or low.
Total Error = Effect seen when random and systematic errors are combined.
xi' of yi = Individual observations.
Y-intercept (a, y0) = The y value when x = 0.
1. Campbell, J.M., and Campbell, J.B. 1990. Laboratory Mathematics , 4th ed. Mosby.
2. McClatchey, Kenneth D. 1994. Clinical Laboratory Medicine , Williams & Wilkins.
3. Buttner, J., et al. 1976. Clin. Chem. ; 22:532-540.