The invention provides methods of performing a sizing analysis. In the methods, a sizing ladder used in performing the sizing analysis is corrected. In one method, the sizing ladder is corrected for batch-to-batch variations in a sieving gel. In another method, the sizing ladder is corrected for a sample concentration that is different from the archival sizing ladder concentration. Methods are also provided in which the sizing ladder is corrected using a standard marker in a sample and/or using a real-time standard sizing ladder. The methods may be used individually or in combination.