Method and system for data de-duplication

Comparison of the quality of matching file formats, so as to store those having higher quality in a file de-deduplication system. A transformation matrix (2C3) indicative of a conversion between the file formats are used, such that on receiving a request to store a first file having a first format, if the second format has a higher quality indicator value than the first format and the second format is convertible to the first format deleting the first file. Otherwise the first file is stored file and on receiving a request to retrieve a file with a file format, if the transformation matrix indicates that the format of the stored file is convertible to the format of the requested file, converting the stored file format to the format of the requested file and sending the converted stored file, or otherwise sending the unconverted stored file. Stored files in different formats may be ranked according to quality, which may be based on sampling rate, resolution, compression ratio and an information richness of the content for a media file or an e-mail format.