Survey shows declines in incentive selling. (Outfront: Manage).(Credit Suisse/First Boston)

Wall Street housing stock analyst Ivy Zelman of Credit Suisse/First Boston has come up with an interesting measure to keep her finger on the pulse of the industry. She surveys leading newspapers around the country each week and counts the percentage of ads public home builders place that offer discounts and other incentives to spur sales.

When the percentage of incentive ads begins to climb, it's a sure sign that traffic and sales are softening, or that the end of a quarter is near. Public home builders are notorious for trying to sell, in any way, as many spec houses a