Theories regarding kabe and suji have already been covered, next will be a summarization of the different degree of safety of tiles, making it into a risk table.
The risk ordering above, should be memorized by beginners, because the ordering above decides what tile to discard first when in betaori, which heavily affects deal-in rate.
The table above refers to normal situations, you should consider the following in a real game:
As there will be more points from waiting on the dora, waits on the dora can be predicted. When the dora is chunchan, the deal-in rate is 1.5-1.6 times of the normal rate, while when the dora is a yaochuhai, the deal-in rate increases to 1.8-1.9 times. Also, the tiles near the dora (a.k.a dora soba) have a 10% increase in deal-in rate.
Hence, the degree of safety of the dora tile, yaochuhai's grade has to drop 3 grades, chunchan has to drop 2 grades. While the dora soba has to drop 1 grade.
2) One chance tiles
The degree of safety for one chance tiles is roughly between non suji-pai and suji-pai.
3) The outer tiles of early discarding of 28,37
If the attacker discarded 28, 37 number tiles early, the outer tiles will be safer than usual. (For example if the player who declared riichi discarded 8 sou, then 9 sou is relatively safe.) Deal-in rate is 30% lower than usual.
Also, there are many discard reading books about ura suji, aida yon ken, matagi suji on the market. You might be wondering, why hasn't there been any talk about discard reading.
The reason is because according to game record statistics from 東風莊's super high level tables, this so called discard reading, do not affect the risk of related tiles. So when you're in betaori, it's better to ignore these "skills".