The Volume effect and the Tonality effect doesn’t explain everything because sometimes even if weights are equals (volume effect = 0) new conversation may arise.
The new weight variation (on the current period) has also a new tonality that is totally different from the previous period (ex: if a new population answer now and not before) and that is not catchable with both effects that we have seen previously.
For each category:
Mix effect = Delta E-index (of the 2 periods) x Delta Weight (of the 2 periods)