For the last 20 years, I have been doing Behavioural Analysis on Sales people. That is I sit silently in the background, observing their and their Client's Verbal Behaviours. I record these by categories of Seeking Information and Giving Information, i.e. Asking Questions or Talking. I have recorded more than 3,000 face-to-face sales calls; the average length of these was 38 minutes (shortest 2 minutes, longest 4 hours and 57 minutes!).
When you map Verbal Behaviour to Sales Success then a Golden ration of 4:1 (a Pareto 80:20) appears. The most successful sales people spend 4 times as much time Asking Questions as they do Giving Information. You would need FOUR Ears for every Mouth, not the oft quoted "two ears and one mouth". Less than the golden ratio appears to be much less effective. More than the golden ratio, it feels like an interrogation and again is far less effective.
It is, of course, not as simple as ratios. It is also about Question types, Rhetorical, Leading and Multiple questions are ineffective. In addition, there is no discernable difference between Open and Closed Questions they vary in effect with the Client type.
SPIN® can be used as a basis of 'problem/solution' selling as per the question.
However, 'opportunity/proposition' selling uses a different basis for question constructs.
Perhaps an even more interesting point of view is "How many questions does a Client ask before Buying?" …..
I have the answer to that too.