def accuracy_score(truth, pred):
    """ Returns accuracy score for input truth and predictions. """

    # Ensure that the number of predictions matches number of outcomes
    if len(truth) == len(pred):

        # Calculate and return the accuracy as a percent
        return "Predictions have an accuracy of {:.2f}%.".format((truth == pred).mean()*100)

    else:
        return "Number of predictions does not match number of outcomes!"