Convert forward returns to returns relative to mean
    period wise all-universe or group returns.
    group-wise normalization incorporates the assumption of a
    group neutral portfolio constraint and thus allows allows the
    factor to be evaluated across groups.

    For example, if AAPL 5 period return is 0.1% and mean 5 period
    return for the Technology stocks in our universe was 0.5% in the
    same period, the group adjusted 5 period return for AAPL in this
    period is -0.4%.

    Parameters
    ----------
    factor_data : pd.DataFrame - MultiIndex
        Forward returns indexed by date and asset.
        Separate column for each forward return window.
    grouper : list
        If True, demean according to group.

    Returns
    -------
    adjusted_forward_returns : pd.DataFrame - MultiIndex
        DataFrame of the same format as the input, but with each
        security's returns normalized by group.

def demean_forward_returns(factor_data, grouper=None):
    """
    Convert forward returns to returns relative to mean
    period wise all-universe or group returns.
    group-wise normalization incorporates the assumption of a
    group neutral portfolio constraint and thus allows allows the
    factor to be evaluated across groups.

    For example, if AAPL 5 period return is 0.1% and mean 5 period
    return for the Technology stocks in our universe was 0.5% in the
    same period, the group adjusted 5 period return for AAPL in this
    period is -0.4%.

    Parameters
    ----------
    factor_data : pd.DataFrame - MultiIndex
        Forward returns indexed by date and asset.
        Separate column for each forward return window.
    grouper : list
        If True, demean according to group.

    Returns
    -------
    adjusted_forward_returns : pd.DataFrame - MultiIndex
        DataFrame of the same format as the input, but with each
        security's returns normalized by group.
    """

    factor_data = factor_data.copy()

    if not grouper:
        grouper = factor_data.index.get_level_values('date')

    cols = get_forward_returns_columns(factor_data.columns)
    factor_data[cols] = factor_data.groupby(grouper)[cols] \
        .transform(lambda x: x - x.mean())

    return factor_data