Symbol pi

The passage around this formula

…ordinary case for a general-purpose product, precision — the share of flagged requests that are actually harmful — is $\text{Precision} = \frac{\mathrm{TPR} \cdot \pi}{\mathrm{TPR} \cdot \pi + \mathrm{FPR} \cdot (1 - \pi)}$. where $\mathrm{TPR}$ and $\mathrm{FPR}$ are the true- and false-positive rates and $\pi$ is the base rate. Plugging in $\mathrm{TPR}$ = 0.98 , $\mathrm{FPR}$ = 0.01 and $\pi$ = 0.0005 gives a precision of roughly 4.7%: more than nineteen out of twenty flagged requests are false alarms, even though the classifier’s headline accuracy sounds close to perfect. This is not a defect specific…

Read this part in the article →