As mentioned before, an appraiser can generally choose from three approaches to determine value. One or two of these approaches will usually be most applicable, with the other approach or approaches usually being less useful. The appraiser has to think about the "scope of work", the type of value, the property itself, and the quality and quantity of data available for each approach. No overarching statement can be made that one approach or another is always better than one of the other approaches.
The sales comparison approach is based primarily on the principle of substitution. This approach assumes a prudent (or rational) individual will pay no more for a property than it would cost to purchase a comparable substitute property. The approach recognizes that a typical buyer will compare asking prices and seek to purchase the property that meets his or her wants and needs for the lowest cost. In developing the sales comparison approach, the appraiser attempts to interpret and measure the actions of parties involved in the marketplace, including buyers, sellers, and investors.