I want to share this:
As I am learning for my state medical examination, I am trying to power through all my medical Anki cards that I have amassed over 2 years.
In my medical cards collection of 67,951 cards, which have not all been made at the same time but through a continuous and painful process, 9743 cards have failed. This translates into a true retention of 85.6%. My desired retention is set at 95%. This process has taken place at an average of 3000 cards per day so I don’t know if this anecdote is of significance since it could be influenced by lots of factors (time of the day, difficulty, me studying in a foreign language, backlog size). I am also using different presets for different subdecks, so I don’t know if that plays a role.
But I can’t help but think the discrepancy is too large for it to be just due to some “noise”. I think I made a similar post before but I think I know at least one reason behind this. Interference. As new knowledge is acquired it either strengthens, is indifferent, or more often overlaps and interferes with old knowledge.
I don’t know exactly how FSRS deals with interference from the continuous growth of knowledge (from what I can tell, it is suboptimal), but it is a real thing. That is just the retrograde interference, there is only the anterograde one, whereby old knowledge interferes with acquiring new knowledge.
If FSRS could somehow manage to model interference based on the card difficulty of new cards and then change the card difficulty of other cards based on it, then I believe that would be a dealbreaker.