First: Errors of judgment in appraising the value of the property. Since each piece of real estate stands by itself, there can never be a "market value" for it in the sense that there is for bonds or shares of stock, where each sale is representative of the value of the entire issue. The valuation of real estate must rest on opinion only, and while it may be comparatively easy for an expert with full information to value real estate correctly in an active market, in a market where transactions are few the difficulty is very great. In order to have appraisals of any value, a real estate expert must have at his command a large fund of information in regard to sales of property, rentals of property and the cost of construction of buildings, since these are indispensable to a proper valuation of real estate. It is not always easy to obtain information in regard to the consideration for sales, especially in New York City, where the practice is growing of setting out a nominal consideration of one dollar in deeds conveying property. The insertion in deeds of fictitious considerations must also be guarded against, such considerations being sometimes met with where the amount has been placed below the actual selling price, in the hope of obtaining a lower assessment for purposes of taxation, and more frequently placed at a figure above the selling price in the hope of giving the property a fictitiously high value.