  We use numerical simulations based on an extended Lugiato-Lefever equation
(LLE) to investigate the stability properties of Kerr frequency combs generated
in microresonators. In particular, we show that an ensemble average calculated
over sequences of output fields separated by a fixed number of resonator
roundtrips allows the coherence of Kerr combs to be quantified in terms of the
complex-degree of first-order coherence. We identify different regimes of comb
coherence, linked to the solutions of the LLE. Our approach provides a
practical and unambiguous way of assessing the stability of Kerr combs that is
directly connected to an accessible experimental quantity.
