def get_split_ratio(df):
    """
    This function returns a copy of a data frame with a new column
    for split ratios.
    :param dataframe df: contains split times for individual runners
    :return dataframe df_new: a copy of the passed data frame which contains one
    new column; the split ratio comparing the second half of each individual's
    marathon to the first half.
    """
    df_new = df.copy()
    df_new["split_ratio"] = (df_new["official_time"] - df_new["half"]) / (
        df_new["half"]
    )
    return df_new