Market based data cleaning for mitigation of idiosyncratic errors in transaction data used for property valuation. The market based data cleaning technique helps to ensure that the most accurate record is retained for a transaction when duplicate records are found, by ensuring that the retained record is the most consistent with other transactions of the same property, the local market trend, and neighborhood market. Algorithms accommodate the adoption of a value as a representative single record for a transaction where multiple records are present for a transaction. Following duplicate removal, market based data cleaning further eliminates erroneous records by eliminating transaction outliers, also preferably based upon the local market trend, along with all of the remaining transactions for each given property.