The good news is your home may be worth more than the rock-bottom price that your neighbors' houses fetched. The bad news: No one but you might think so.

The one point of widespread agreement in the real-estate industry is that there is no single accurate index of home prices. They are all over the map, cover different sets of homes and may exclude parts of the country or be unduly influenced by the mix of homes sold in a given month.

As...