def most_common(data, index):
    """
    >>> most_common(['111', '110', '100', '000'], 0)
    '1'
    >>> most_common(['111', '110', '100', '000'], 1)
    >>> most_common(['111', '110', '100', '000'], 2)
    '0'
    """
    middle = len(data) / 2
    ones = [number[index] for number in data].count('1')

    if ones > middle:
        common = '1'
    elif ones < middle:
        common = '0'
    else:
        # None means 1 and 0 most equally common
        common = None

    return common