def ppv_from_df(merged_df, number_to_test, length_cutoff=8):
    """
    Pretty straightforward calculation of positive predictive value given
    the merged_df input, n to test, and what to define as a contact (in
    units of angstroms)
    """
    temp_df = merged_df[:number_to_test]
    tps = temp_df[temp_df['distance']<=length_cutoff]['distance'].count()
    totals = temp_df['distance'].count()
    return  tps/totals, totals