δ-presence metric, used to estimate how likely it is for an attacker to
figure out that one given individual appears in a de-identified dataset.
Similarly to the k-map metric, we cannot compute δ-presence exactly
without knowing the attack dataset, so we use a statistical model
instead.
ISO 3166-1 alpha-2 region code to use in the statistical
modeling. Required if no column is tagged with a region-
specific InfoType (like US_ZIP_5) or a region code.