Page "Hash function" Paragraph 49

These simple formulas will not do if the input values are not equally likely, or are not independent.

For instance, most patrons of a supermarket will live in the same geographic area, so their telephone numbers are likely to begin with the same 3 to 4 digits.

In that case, if n is 10000 or so, the division formula ( z × n ) ÷ N, which depends mainly on the leading digits, will generate a lot of collisions ; whereas the remainder formula z mod n, which is quite sensitive to the trailing digits, may still yield a fairly even distribution.