Probabilities for getting unchanging vs. changing hexagrams

I've read in a few places the probabilities of getting different lines using different methods: yarrow stalk (or 16 objects), or three coins. For example, that the probability of getting an 8 is three out of sixteen, or of getting a 9 is 2 out of sixteen ....

But what about for the hexagrams themselves? Does anyone know the probability of getting an unchanging hexagram versus a changing one? One third of the time? Three out of sixteen? One in twenty? .....

Best, D.