def _select_ps(p):
    """returns the points to use for interpolating p"""
    if p >= 0.99:
        return 0.99, 0.995, 0.999
    elif p >= 0.975:
        return 0.975, 0.99, 0.995
    elif p >= 0.95:
        return 0.95, 0.975, 0.99
    elif p >= 0.9125:
        return 0.9, 0.95, 0.975
    elif p >= 0.875:
        return 0.85, 0.9, 0.95
    elif p >= 0.825:
        return 0.8, 0.85, 0.9
    elif p >= 0.7625:
        return 0.75, 0.8, 0.85
    elif p >= 0.675:
        return 0.675, 0.75, 0.8
    elif p >= 0.5:
        return 0.5, 0.675, 0.75
    else:
        return 0.1, 0.5, 0.675
