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

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?

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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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?

Yes, it is covered in the manual.

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%

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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