def get_broca_index_diff(df):
    """
    The Broca Index is an estimation of ideal body weight using a heigh
    measurement only.

    Here I return the person's actual weight - the broca index weight.

    weight in kg and height in cm
    Ref.: https://www.topendsports.com/testing/tests/broca-index.htm
    """

    df["BI_diff"] = df["weight"] - (df["height"] - 100)
    return df