code: def mean_otu_pct_abundance(ra, otuIDs):
      
      sids = ra.keys()
      otumeans = defaultdict(int)
  
      for oid in otuIDs:
          otumeans[oid] = sum([ra[sid][oid] for sid in sids
                               if oid in ra[sid]]) / len(sids) * 100
      return otumeans
docs: Calculate the mean OTU abundance percentage.
  
      :type ra: Dict
      :param ra: 'ra' refers to a dictionary keyed on SampleIDs, and the values are
                 dictionaries keyed on OTUID's and their values represent the relative
                 abundance of that OTUID in that SampleID. 'ra' is the output of
                 relative_abundance() function.
  
      :type otuIDs: List
      :param otuIDs: A list of OTUID's for which the percentage abundance needs to be
                     measured.
  
      :rtype: dict
      :return: A dictionary of OTUID and their percent relative abundance as key/value pair.