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)