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SensSpec.demo <- function(sens, spec, prev, n=100000, step=11) {
mat <- matrix(NA, ncol=4, nrow=4)
dimnames(mat) <- list( Test=c('Positive','Negative','','Total'),
Disease=c(' Yes',' No',' ',' Total') )
pplines <- c(' ',
'PPV =',
'NPV =')
mat[4,4] <- n
if(step>1){
mat[4,1] <- round(n*prev)
}
if(step>2){
mat[4,2] <- n-mat[4,1]
}
if(step>3){
mat[1,1] <- round( sens*mat[4,1] )
}
if(step>4){
mat[2,1] <- mat[4,1] - mat[1,1]
}
if(step>5){
mat[2,2] <- round( spec*mat[4,2] )
}
if(step>6){
mat[1,2] <- mat[4,2]-mat[2,2]
}
if(step>7){
mat[1,4] <- mat[1,1]+mat[1,2]
}
if(step>8){
mat[2,4] <- mat[2,1]+mat[2,2]
}
if(step>9){
pplines[2] <- paste( 'PPV = ', mat[1,1], '/', mat[1,4],
' = ', round(mat[1,1]/mat[1,4], 4), sep='')
}
if(step>10){
pplines[3] <- paste( 'NPV = ',mat[2,2], '/', mat[2,4],
' = ', round(mat[2,2]/mat[2,4], 4), sep='')
}
print(mat, na.print='')
cat(paste(pplines, collapse='\n'),"\n\n")
invisible(mat[-3,-3])
}
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