def load_factor(ts, resolution=None, norm=None):
    """
    Calculate the ratio of input vs. norm over a given interval.

    Parameters
    ----------
    ts : pandas.Series
        timeseries
    resolution : str, optional
        interval over which to calculate the ratio
        default: resolution of the input timeseries
    norm : int | float, optional
        denominator of the ratio
        default: the maximum of the input timeseries

    Returns
    -------
    pandas.Series
    """
    if norm is None:
        norm = ts.max()
    if resolution is not None:
        ts = ts.resample(rule=resolution).mean()
    lf = ts / norm
    return lf
