def create_index(people):
    """
    Create an index data structure to efficiently retrieve people with similar
    characteristics.
    :param people: list of people
    :return: dict (blocking key -> list of matching people)
    """
    index = {}
    for p in people:
        x = p.bkey
        if x not in index:
            index[x] = [p]
        else:
            index[x].append(p)
    return index