Cities like San Diego or Tampa are improving in the housing affordability index because buying activity in the lower segment of the market has picked up.

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The housing affordability index (HAI) measures whether a family earning a median income could qualify for a mortgage loan on a median-priced, single-family home.

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Historically low interest rates and bottom-of-the-trough housing prices have combined to increase the Housing Affordability Index (as reported by the National Association of Realtors) to record highs.

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We took the job-growth and cost-of-living rankings, and added to the mix a housing affordability index from research firm Moody's Economy.com and a salary index from Seattle-based compensation collection firm PayScale .

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The housing affordability index gauges the amount of a local, median-priced home (the price at which 50% of homes are more expensive and 50% are less expensive) you can buy if you earn the local median income, given current interest rates.

FORBES: Most Overpriced Places In The U.S. 2006

Starter-home affordability: Moody's Economy.com provided us with an index of housing affordability based on the cost of a median priced home in each city compared with the median income.

FORBES: Special Report

Use the Case-Shiller index, where the affordability of housing worsened sharply during the boom, and mortgage payments are still high in relation to incomes.

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