The use of spaced repetition memory systems has changed my life over the past couple of years. Here's a few things I've found helpful:
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Yes, when a card later seems poorly chosen. It's not common - I doubt I've deleted more than 1 or 2% of the cards. As to why: because it makes a large difference, continually noticeable (and accumulating over time), with nearly all my other work, for a small investment of time
