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Biometric Systems/final notes/2. Performance indexes.md

@@ -98,3 +98,39 @@ Of course templates should be different, not computed i.e. by frames of the same
 > - pj does not belong to the gallery (most trivial)
 > - pj belongs to an enrolled subject but the probe claimed another identity, not the real one.
 
+What if ERR in two systems is the same, but the curves are different?
+
+We can use ROC curve or DET curve.
+For ROC, we can compute the area below the curve and use it as a metric, the higher the better.
+
+#### Possible errors: identificaiton - open set
+In an open set identification task, the system determines if the individual's biometric signature matches a signature of someone in the gallery.
+The individual **does not make** and identity claim.
+- More possible error situations, depending on the matcher and on the threshold
+- A problem may occur if the system returns more possible candidates below the threshold. Who is the right one?
+> [!PDF|yellow] [[LEZIONE2_Indici_di_prestazione.pdf#page=27&selection=0,8,9,8&color=yellow|LEZIONE2_Indici_di_prestazione, p.27]]
+> > Possible errors: identification – open set
+> 
+> 
+
+correct detect and identify rate = rate over which the correct individual has the identified score and so is identified correctly.
+
+false alarm rate = rate over which unenrolled users are identified as another user in the db.
+
+We compute that by testing the system with lots of probes belonging to set Pg if enrolled or set Pn if not.
+
+
+We define
+- rango(pj) = the position in the list where the first template for the correct identity is returned
+- DIR (at rank k) (Detection and Identication Rate (at rank k)): the probability of correct identification at rank k (the correct subject is returned at position k)
+- The rate between the number of individuals correctly recognized at rank k and the number of probes belonging to individuals in PG
+- If identification does NOT happen at rank 1, we have a False Reject.
+- FRR or more specifically FNIR (False Reject Rate or False Negative Identification Rate): the probability of false reject expressed as 1 - DIR (at rank 1)
+
+- FAR or more specifically FPIR (False Acceptance Rate or False Positive Identification Rate) or False Alarm Rate (Watch List): the probability of false acceptance/alarm 
+- The rate between the nuber of impostor recognized by error and the total number of impostors in PN
+
+
+#### Closed set
+We don't have thresholds!
+The only possible error is that the correct identity does not appear at rank 1.