def get_word_size(similarity_threshold):
    """
    http://www.bioinformatics.org/cd-hit/cd-hit-user-guide.pdf

    -n 5 for thresholds 0.7 ~ 1.0
    -n 4 for thresholds 0.6 ~ 0.7
    -n 3 for thresholds 0.5 ~ 0.6
    -n 2 for thresholds 0.4 ~ 0.5
    """
    if similarity_threshold >= 0.7:
        return 5
    elif similarity_threshold >= 0.6:
        return 4
    elif similarity_threshold >= 0.5:
        return 3
    else:
        return 2