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Just a note

If the test were capable of perfect discrimination, progressively reducing the cut-off value would make the ROC curve pass up to the top left hand corner, and then across to the top right hand corner.

    In other words:
  • At the optimal cut-off point in the top left hand corner, the sensitivity and specificity would both be 100% - all positive individuals would test positive and all negative individuals would test negative.
  • Above the optimal cut-off point the specificity would be 100%, but the sensitivity would decrease as the cut-off point was further increased.
  • Below the optimal cut off point the sensitivity would be 100%, but the specificity would decrease as the cut-off point was further reduced.

At the other extreme, if the test were completely useless, the ROC plot would yield a straight line (shown here in dots).