Which scale for parton densities? Highlight from $k_{t}$-factorization

Using the relationship between collinear and $k_t$-factorization, we discuss the hard scale uncertainty. While the $k_t$-factorization is free of this uncertainty, it should be taken into account in the case of collinear factorization. Contrary to the factorization scale uncertainty, the hard scale uncertainty is not reduced by higher order calculations.