An index of housing affordability declined.

The National Assn. of Realtors said its affordability index fell to 97.8% in October from a six-year high of 98.3% in September. The index, which measures the ability of a typical family to afford a home, means that a family earning the annual median income of $27,689 would have had 97.8% of the income needed to qualify for an existing home costing $75,600. Clark E. Wallace, president of the realtors group, said the affordability index closely follows the rise and fall of mortgage rates.