A method of analyzing the composition of a microbiome based on taxonomic variation in the DNA sequence of the microbial 16S-23S rRNA internal transcribed spacer regions in the genomic DNA of the microorganisms in said microbiome, wherein the sequences of conserved DNA regions comprised in the 16S and 23S rRNA sequences flanking said ITS region in the genomic DNA of said microorganisms comprise primer binding sites for amplification of said ITS regions, said method comprising the steps of: a) providing a sample of genomic DNA from a microbiome; b) providing a PCR calibrator system, comprising a set of PCR amplification primers at least one of which primers comprises a label, and a set of at least two PCR calibrators, each PCR calibrator consisting of a DNA fragment comprising a spacer region having a DNA sequence of a given length flanked by upstream and downstream adapter DNA sequences that comprise primer binding sites for binding of said PCR amplification primers wherein said set of PCR amplification primers is for PCR amplifying the spacer region DNA sequence of all PCR calibrators in said set of at least two PCR calibrators, wherein the spacer region DNA sequence comprised in each of said PCR calibrators in said set of at least two PCR calibrators is of a different length, and wherein each PCR calibrator in said set of at least two PCR calibrators is present in equal amount or in a known amount relative to other PCR calibrators in said set; c) adding said set of at least two PCR calibrators from said PCR calibrator system to said sample of genomic DNA; d) performing a PCR amplification reaction on said sample of genomic DNA comprising said set of at least two PCR calibrators using said set of PCR amplification primers from said PCR calibrator system as a first set of amplification primers to amplify and provide amplification products of said ITS region(s) comprised in said set of at least two PCR calibrators, and using at least a second set of PCR amplification primers directed to said flanking conserved DNA regions to thereby co-amplify and provide amplification products of said ITS regions comprised in said sample of genomic DNA, and; e) providing a standard curve by determining the PCR amplification efficiency of each of said at least two PCR calibrators from said PCR calibrator system in said PCR amplification reaction of step d) and expressing said PCR amplification efficiency as a function of the length of the DNA sequence of the ITS region; f) determining the length-specific amplification efficiency for ITS regions of different length comprised in said genomic DNA sample and amplified in step d) using the standard curve as provided in step e); g) determining the abundance of microbial 16S-23S rRNA internal transcribed spacer regions of different length in said microbiome using the length-specific amplification efficiencies determined in step f), and h) analyzing the composition of a population of microorganisms based on the abundances of ITS regions of different length determined in step g). a) providing a sample of genomic DNA from a microbiome; b) providing a PCR calibrator system, comprising a set of PCR amplification primers at least one of which primers comprises a label, and a set of at least two PCR calibrators, each PCR calibrator consisting of a DNA fragment comprising a spacer region having a DNA sequence of a given length flanked by upstream and downstream adapter DNA sequences that comprise primer binding sites for binding of said PCR amplification primers wherein said set of PCR amplification primers is for PCR amplifying the spacer region DNA sequence of all PCR calibrators in said set of at least two PCR calibrators, wherein the spacer region DNA sequence comprised in each of said PCR calibrators in said set of at least two PCR calibrators is of a different length, and wherein each PCR calibrator in said set of at least two PCR calibrators is present in equal amount or in a known amount relative to other PCR calibrators in said set; c) adding said set of at least two PCR calibrators from said PCR calibrator system to said sample of genomic DNA; d) performing a PCR amplification reaction on said sample of genomic DNA comprising said set of at least two PCR calibrators using said set of PCR amplification primers from said PCR calibrator system as a first set of amplification primers to amplify and provide amplification products of said ITS region(s) comprised in said set of at least two PCR calibrators, and using at least a second set of PCR amplification primers directed to said flanking conserved DNA regions to thereby co-amplify and provide amplification products of said ITS regions comprised in said sample of genomic DNA, and; e) providing a standard curve by determining the PCR amplification efficiency of each of said at least two PCR calibrators from said PCR calibrator system in said PCR amplification reaction of step d) and expressing said PCR amplification efficiency as a function of the length of the DNA sequence of the ITS region; f) determining the length-specific amplification efficiency for ITS regions of different length comprised in said genomic DNA sample and amplified in step d) using the standard curve as provided in step e); g) determining the abundance of microbial 16S-23S rRNA internal transcribed spacer regions of different length in said microbiome using the length-specific amplification efficiencies determined in step f), and h) analyzing the composition of a population of microorganisms based on the abundances of ITS regions of different length determined in step g).