Edge Waits
Something to note is that penchan (Edge waits, like 
or 
) are really hard to complete – not only is there only one tile, but you have to upgrade the wait twice > 
> 
before it gets any better.
Single tiles with lots of room around them (for example, 
), while two tiles away from becoming a set, have lots of ways to get better – they can easily become multi-tile waits (ryanmen, such as 
), or at the very least center waits (
) that upgrade easily to multi-tile waits. In fact, with this hand:
Choosing between discarding vs. or , the difference in number of draws between which sequence completes first is less than 1%.
So, if you have a number of unconnected middle tiles, maybe keeping them all and seeing which one develops is a better idea than keeping that penchan. Try it yourself!