number specifying the threshold for considering a disease outbreak. Should be given as incidence if
<code>population</code> and <code>incidence_denominator</code> are in the <code>tsd</code> object else as cases. It defines the per time-step disease threshold that has to be surpassed for the observation to be
included in the level calculations.</p></dd>


<dt>n_peak</dt>
<dd><p>A numeric value specifying the number of peak observations to be selected from each season in the
level calculations. The <code>n_p