CMatrixLab — Simple

CMatrixLab by Martin Grossman martin.grossman@gmail.com

Your flu test is positive. What’s the chance you really have flu? That’s precision—and it also depends on how common flu is among the people tested.
Compare 10% and 50% sick using the same test. Precision changes, even though the share of sick people correctly testing positive (TPR) and the share of people without flu getting a false alarm (FPR) stay the same.

Sensitivity to the flu signal 50% 0% · Less sensitive 100% · More sensitive How different the two groups are 60% Hard to tell apart Easy to tell apart

Sick share 50% 0% · no one has flu 100% · everyone has flu

TP · Correct detection FP · False alarm FN · Missed TN · Correctly ignored

Full dot or square = 10 people; smaller = partial.

Test −

Test +

Actual flu
500 cases

FN 0

TP 0

Actual no flu
500 cases

TN 0

FP 0

Among positive tests: —

Green: 0 have flu Red: 0 false alarms

1,000 weighted cases. Counts are expected values and may be fractional. Sick share changes counts and precision; TPR and FPR stay fixed at the same threshold and separation.