The analysis of deviant cases—cases that do not fit the general pattern— guard against

The analysis of deviant cases, cases that do not fit the general pattern, helps guard against Selective observation.

Selective observation means Paying attention to situations which support our own conclusion.Selective observation ignores any observations that do not conform to an over generalization.
In the welfare mother example: once you have fashioned your opinion that welfare mothers are lazy, you'll then pay special attention to alternative stories on lazy welfare mothers.