def accuracy(reference, test):
    """
    Calculate the fraction of test items that equal the corresponding reference items.

    Given a list of reference values and a corresponding list of test values,
    return the fraction of corresponding values that are equal.
    In particular, return the fraction of indexes
    {0<i<=len(test)} such that C{test[i] == reference[i]}.

        >>> accuracy(['ADJ', 'N', 'V', 'N'], ['N', 'N', 'V', 'ADJ'])
        0.5

    :param reference: An ordered list of reference values
    :type reference: list
    :param test: A list of values to compare against the corresponding
        reference values
    :type test: list
    :return: the accuracy score
    :rtype: float
    :raises ValueError: If reference and length do not have the same length
    """

    if len(reference) != len(test):
        raise ValueError("Lists must have the same length.")
    num_correct = 0
    for x, y in zip(reference, test):
        if x == y:
            num_correct += 1
    return float(num_correct) / len(reference)