It is the intent of the legislative assembly that local assessors, county directors of tax equalization, and city, township, county, and state boards of equalization use the results of sales, market, and productivity studies as a guide in making assessments and in equalizing assessments of property in this state. The legislative assembly recognizes that sales of property alone provide insufficient information to make accurate judgments concerning the market value of property within the various counties of this state, particularly in view of the limited number of sales which occur within a given period of time in many counties, and that, in order to get an accurate picture of market value, consideration must be given to such factors as property appraisals, productivity, and current usage of property.