A tuple of values for the quasi-identifier columns.
The estimated probability that a given individual sharing
these quasi-identifier values is in the dataset. This value,
typically called δ, is the ratio between the number of records
in the dataset with these quasi-identifier values, and the
total number of individuals (inside and outside the dataset)
with these quasi-identifier values. For example, if there are
15 individuals in the dataset who share the same quasi-
identifier values, and an estimated 100 people in the entire
population with these values, then δ is 0.15.
