These are screening tools. They are not meant to make definitive diagnoses for the most part. They are used for epidemiology and flagging clients for further assessment.
There's always a balance to be made between sensitivity (picking up on real positives) and specificity (not producing false positives). The balance for these instruments is at about 80-85%. If a person is depressed, there's an 80-85% chance they will be correctly classified. If a person is not depressed, there is an 80-85% chance they will be correct classified.
Raise the threshold and you'll be more specific but less sensitive. Lower the threshold and you'll be less specific but more sensitive. It's just how the game works and why questionnaires are only first-pass tools.
There's always a balance to be made between sensitivity (picking up on real positives) and specificity (not producing false positives). The balance for these instruments is at about 80-85%. If a person is depressed, there's an 80-85% chance they will be correctly classified. If a person is not depressed, there is an 80-85% chance they will be correct classified.
Raise the threshold and you'll be more specific but less sensitive. Lower the threshold and you'll be less specific but more sensitive. It's just how the game works and why questionnaires are only first-pass tools.