For an explanation of Sensitivity and Specificity, see the attached file:
"Sensitivity and Specificity (Stokes SAS manual).pdf".
See also the related topic: ROC curves
Wikipedia: http://en.wikipedia.org/wiki/Sensitivity_and_specificity (also attached below)
R code for demonstration
SensSpec.demo {TeachingDemos}
library(TeachingDemos)
for(i in seq(1,11,2)) {
SensSpec.demo(sens=0.95, spec=0.99, prev=0.01, step=i)
if( interactive() ) {
readline("Press Enter to continue")
}
}