What are the chances of each gumball dropping?

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In Sonic 3 (and by extension, Sonic 3 & Knuckles) there is a Gumball-dispenser bonus stage which dispenses different power-ups, including the various shields, rings, extra lives and a few other arena-specific ones, such as the rubber/bounce balls.
You can watch the bonus stage in action here:
What are the chances that the gumball machine dispenses one type of ball over another? Are they uniformly released, or does say, a Bubble shield appear more often then an extra life?
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Sources: Stack Exchange - This article follows the attribution requirements of Stack Exchange and is licensed under CC BY-SA 3.0.
Images: Ron Lach, Anna Tarazevich, Polina Tankilevitch, Pavel Danilyuk