Asset indexes are intended to track changes in the market value of assets. However, some assets such as real estate can change in value for reasons other than that a general change in the market has occurred. For example, a house can have an addition which adds another bedroom or a portion of land can be sub-divided, in which case an increase or decrease respectively in the asset value can occur which is independent of movements in the overall market. A method of generating an asset index under these circumstances is disclosed by effectively removing from the asset portfolio those assets which have had a change in attribute come to notice on the day the asset index is calculated. Such an asset is restored to the portfolio on subsequent days.