Monday, July 16, 2012

Reliability and Validity Cheat Sheet


2 X 2 Contingency Table

Present

Absent Row Total
Positive a (True +) b (False +) a + b
Negative c (False -) d (True -) c + d
Column Total a + c b + d a + b + c+ d

Sensitivity: a/(a+c) The ability of a test to correctly identify people who have the target disorder.

Specificity: d/(b+d) The ability of a test to correctly identify people who do not have the target disorder

Positive predictive value: a/(a+b) The probability that a positive test will correctly identify people who have the target disorder.

Negative predictive value: d/(c+d) The probability that a negative test will correctly identify people who do not have the target disorder.

 • Likelihood ratio of a positive test: [a/(a+c)/1-{d/(b+d)}] : (Sensitivity)/(1-Specificity) A ratio of the probability of a positive test in a person with the condition compared to the probability of a positive test in a person without the condition.

Likelihood ratio of a negative test: [1-{a)/(a+c)}]/[d/(b+d)] : (1-Sensitivity)/(Specificity) A ratio of the probability of a negative test in a person with the condition compared to the probability of a negative test in a person without the condition.

Prevalence: (a+c)/(a+b+c+d) • Accuracy: (a+d)/ (a+b+c+d)

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