A method for obtaining from genomic material genomic copy number information unaffected by amplification distortion, comprising obtaining segments of the genomic material, tagging the segments with substantially unique tags to generate tagged nucleic acid molecules, such that each tagged nucleic acid molecule comprises one segment of the genomic material and a tag, subjecting the tagged nucleic acid molecules to polymerase chain reaction (PCR) amplification, generating tag associated sequence reads by sequencing the product of the PCR reaction, assigning each tagged nucleic acid molecule to a location on a genome associated with the genomic material by mapping the subsequence of each tag associated sequence read corresponding to a segment of the genomic material to a location on the genome, and counting the number of tagged nucleic acid molecules assigned to the same location on the genome having a different tag, thereby obtaining genomic copy number information unaffected by amplification distortion.