While we don't have numbers, and I think it would be almost impossible to quantify anyway, I personally think the average level of writing skill has remained relatively constant over the years.
If that's indeed the case, then the chances are very good that the ratio of good self-published books to bad ones has also remained relatively constant. As more books are published, we get more good ones, more average ones, and more bad ones in roughly the same proportions. Therefore, we can reasonably expect that the number of good self-published books is growing at the same rate as the market.
If that's the case, then what we need to do is see how the rate at which good self-published books get onto the best-seller charts compares to the rate at which the market is growing. If the rate of penetration is increasing faster than the rate at which the field is growing, then the odds would appear to be getting better. If the rate of penetration is increasing more slowly than the field is growing, then the odds are getting worse, even if self-published books are taking an ever-larger piece of the best-seller pie.
Odds are relative numbers, totals are absolute. Without some way to link them you can't derive one from the other, and I've seen no evidence from any of these studies that they've successfully linked the two.
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