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.

def mean_otu_pct_abundance(ra, otuIDs):
    """
    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.
    """
    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