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. 2018 Sep 15;15(9):2018. doi: 10.3390/ijerph15092018

Table 1.

Formula for Sensitivity, Specificity, Positive Predictive Value and Negative Predictive Value.

Variables Used SD Non-SD Total
Presence of WS a b a + b
Absence of WS c d c + d
Total a + c b + d a + b + c + d
Example of the calculation
Sensitivity = a/(a + c)
Specificity = d/(b + d)
PPV = a/(a + b)
NPV = d/(c + d)
Example: For any number of warning signs, a = 31, b = 300, c = 3, d = 366.
   For one warning sign only, a = 12, b = 155, c = 22, d = 511
    For two warning signs only, a = 11, b = 105, c = 23, d = 561