criterion validity worksheet
TRANSCRIPT
Worked ExampleWorked Example
Refer to Worksheet 1
Dr. Alaa Hassan - MB BCh, MSc (PH), MD
• Lets calculate the Positive and Negative Predictive Values of a test which diagnoses Coronary Heart Disease.
• The test has a – Sensitivity 80%– Specificity 75%
Dr. Alaa Hassan - MB BCh, MSc (PH), MD
Scenario 1
• Lets Apply the test on a sample of 100 patients of whom 20% are known to have Coronary Heart Disease (Prevalence of the disease is 20%)
• Lets try to draw the summary table we used in the lung cancer example.
• This table is called 2X2 table.
Dr. Alaa Hassan - MB BCh, MSc (PH), MD
DiseasedNot
Diseased
Step 1: find the number of diseased and non diseased individuals
• Since the prevalence of disease is 20% so The number of diseased individuals =
• total sample X20/100
• 100 X20/100 = 20
• The number of Disease free individuals can be obtained by subtraction
• 100 – 20 = 80
Dr. Alaa Hassan - MB BCh, MSc (PH), MD
DiseasedNot
Diseased
Test Positive
?
Test Negative
?
20 80
Step 2: Calculate true Positives
This is the proportion of Diseased who will be test Positive
= Number of diseased X Sensitivity
= 20X80/100=16
Dr. Alaa Hassan - MB BCh, MSc (PH), MD
Step 3: Calculate true Negatives
This is the proportion of Disease free who will be test Negative
= Number of Diseased X Sensitivity
= 80X75/100=60
DiseasedNot
Diseased
Test Positive
?
Test Negative
?
20 80
Dr. Alaa Hassan - MB BCh, MSc (PH), MD
Step 4: Calculate False Positives and False Negatives
By SubtractionAll Disease free – True Negatives
= False Positives80 – 60= 20
DiseasedNot
Diseased
Test Positive
?
Test Negative
?
20 80All Diseased – True positives =
False Negatives20 -16 =4
Dr. Alaa Hassan - MB BCh, MSc (PH), MD
Step 5: Calculate all test positives and all test negatives by Addition
True positives + False Positives = All test Positives
= 16 + 20= 36Diseased
Not Diseased
Test Positive
Test Negative
20 80True Negatives + False Negatives
=All test negatives= 60 + 4 = 64
Dr. Alaa Hassan - MB BCh, MSc (PH), MD
Step 6: Calculate Positive and Negative Predictive Value
Negative Predictive Value=True negatives/ all test
negativesX10060/64X100
93.75%
Positive Predictive Value =True positives / All Positives X10016/36 X100
44.44%
DiseasedNot
Diseased
Test Positive
Test Negative
20 80
Dr. Alaa Hassan - MB BCh, MSc (PH), MD
• Now , Let’s See what happens if we apply the same test to a sample of 100 individuals , but the prevalence of disease is 40%
• Can we Calculate the Positive and Negative predictive values in this new setting?
Dr. Alaa Hassan - MB BCh, MSc (PH), MD
DiseasedNot
Diseased
Step 1: find the number of diseased and non diseased individuals
• Since the prevalence of disease is 40% so The number of diseased individuals =
• total sample X40/100
• 100 X40/100 = 40
• The number of Disease free individuals can be obtained by subtraction
• 100 – 40 = 60
Dr. Alaa Hassan - MB BCh, MSc (PH), MD
DiseasedNot
Diseased
Test Positive
?
Test Negative
?
40 60
Step 2: Calculate true Positives
This is the proportion of Diseased who will be test Positive
= Number of diseased X Sensitivity
= 40X80/100=32
Dr. Alaa Hassan - MB BCh, MSc (PH), MD
Step 3: Calculate true Negatives
This is the proportion of Disease free who will be test Negative
= Number of Diseased X Sensitivity
= 60X75/100=45
DiseasedNot
Diseased
Test Positive
?
Test Negative
?
40 60
Dr. Alaa Hassan - MB BCh, MSc (PH), MD
Step 4: Calculate False Positives and False Negatives
By SubtractionAll Disease free – True Negatives
= False Positives60 – 45= 15
DiseasedNot
Diseased
Test Positive
?
Test Negative
?
40 60All Diseased – True positives =
False Negatives40 -32 =8
Dr. Alaa Hassan - MB BCh, MSc (PH), MD
Step 5: Calculate all test positives and all test negatives by Addition
True positives + False Positives = All test Positives
= 32 + 15= 47Diseased
Not Diseased
Test Positive
Test Negative
40 60True Negatives + False Negatives
=All test negatives= 45 + 8 = 53
Dr. Alaa Hassan - MB BCh, MSc (PH), MD
Step 6: Calculate Positive and Negative Predictive Value
Negative Predictive Value=True negatives/ all test
negativesX10045/53X100
84.90%
Positive Predictive Value =True positives / All Positives X10032/47 X100
68.08%
DiseasedNot
Diseased
Test Positive
Test Negative
40 60
Dr. Alaa Hassan - MB BCh, MSc (PH), MD
Dr. Alaa Hassan - MB BCh, MSc (PH), MD
• Predictive value of a diagnostic test , depends on the Prevalence of the disease among the population.
• If you use a test developed and tested on a different population bare in mind the predictive value will be different.