see the Microbiome Quality Control Project (MBQC) [38]), this issue must still be actively evaluated to assess the potential for technical biases. Host information, such as age and sex, are nearly prohibitive to access, requiring dean-level signatures for each individual piece of metadata, which makes explaining any systematic patterns in the data impossible without knowing in advance what pattern one expects to see. The end result is that use of the HMP 16S rRNA as a robust reference set has proven difficult.
