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.
REFERENCES
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.
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