This field comparator compares the two fields (or field lists) given
to it as strings character-wise, with a maximum number of different
characters that can be tolerated. This number has to be set with the
argument `max_key_diff`

. If the number of different characters
is larger than zero but equal to or smaller than `max_key_diff`

the partial agreement weight is calculated using the following
formula.

If the number of key differences is larger than

`max_key_diff`

then the disagreement weight is returned.
This field comparator can also be used to compare numerical fields, such as date of birth, telephone numbers, etc.

If a frequency table is given for this field comparator, the agreement weight will be calculated using the frequency of the value in the input fields that are compared, as described in Section 9.2.1. Frequency dependent weights will also be used to calculate a partial agreement weight.