How does the Anki Algorithm with an ivl mod less than 100% not shrink the interval on Good button presses?

Scheduling
1

My understanding of the algorithm for the good button is:
NewIvl = ivl * Ease * ivlModifier * fuzz

So I had a card today that had an ivl of 1.88 years (686.2 days), an ease of 250%, and a ivl modifier of 25%.

So if I know how to plug in numbers (and I may not):
NewIvl = 686.2 * 2.5 * .25 * 1(we can assume fuzz is approximately 1)
Which gives me a smaller ivl of 428.875 days, despite me answering good.
That’s not what happened though, Anki gave it a new ivl of 1.89 years (689.85 days).

So can anyone clear up for me where I screwed up my math?

2

Anki is designed so that Good never decreases the interval below its previous value. Such a low multiplier was not intended to be supported, and recent Anki versions will ignore values below 50%. If your goal is to reduce the interval on some cards, you can either use the ‘set due date’ feature in recent versions, or (IIRC) reduce the hard interval.

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3

Ah. Thanks Dae! I had a deck that wasn’t sticking with large intervals and eventually lowered it to 25. Is the behavior you mentioned documented anywhere yet?

4

Yes, it is covered in the manual.

5

Hey Dae,
Recently I was looking into this again, as I have a deck that has low retention and has hoping to try out a very low ivl modifier. I’m still trying to understand how Anki will schedule my cards with low ivl modifiers, though I can’t find any more information in the manual about:

A low multiplier was not intended to be supported, and recent Anki versions will ignore values below 50%

6

One final thing to note is that Anki forces a new interval to be at least 1 day longer than it was previously, so that you do not get stuck reviewing with the same interval forever. If your goal is to repeat a card once a day for multiple days, you can do that by setting more learning mode steps, instead of by adjusting this modifier.

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