The easy answer is when there are too many adjustments. However, selecting good comparable sales in the boroughs of New York City is easier, in my opinion, then in the suburban sprawl in Nassau and Suffolk County. When the geographical area contains developments, similar lot sizes and house designs, then the selection can also be easier. Its when the improved sites have different designs, lot sizes, views, etc. that the appraiser must be knowledgeable when selecting the right comparable sales. Such is appraising on the eastern end of Long Island. Granted there are times when there are a considerable amount of sales and selecting the correct comparable sale is easy for the professional. But when sales are down it becomes more difficult to select the proper comparable sales. Only a seasoned Certified and Licensed State Appraiser should be employed. Their knowledge of the present real estate market as well as their ability to make proper adjustments based on their historical appraisals including paired market analysis, enables them to estimate true market value. Remember, what some consider comparable sales are really not comparable. The appraiser's clients will be able to understand why the appraiser selected the comparable sales to adjust for there differences and why the appraiser estimated the fair market value for their property.