The scaling factors adjust the difference in scale between V N (d) and DF (w, d).
the α parameter determines the best ratio between V N (d) and DF (w, d) for every used dataset; a higher (lower) α means that a larger (smaller) number of types are included in h. We select the α that gives the highest inter-quartile range (iqr), from 0.1 to 3.0 incremented by 0.1, because large variances between documents are desirable for our purposes.