The appraisal district must repeat its appraisal process for property at least once every three years. To save time and money, the appraisal district uses mass appraisal to appraise large numbers of properties. In a mass appraisal, the district first collects detailed descriptions of each taxable property in the district. It then classifies properties according to a variety of factors, such as size, use, and construction type. Using data from recent property sales, the district appraises the value of typical properties in each class. Taking into account differences such as age or location, the district uses “typical” property values to appraise all the properties in each class.