What are the chances of unboxing a StatTrak weapon?

If we relate to the model that is shown to us when we unbox items, i think that the possible listings are shuffled and then laid in a random order but with the very valuable items in the extremities of the list. I think when a case is opened, a random number is generated somehow that it has some kind of an average number, and average weapons standing in the middle of the list. From what i've seen on many unbox videos on youtube, if the roll is higher (you get further in the list), there is a greater chance to get a better weapon. However there are still some common ones past the "middle" area of the list.

I think nothing can be said certainly, just as in Dota 2, there is no certainty that you will get at least a mythical (or unusual) from a chest with lots of rares.