# I made a mistake in my predictions for/on/by/in 42 days?

Let's say that in April I had been predicting a fall of stocks for May 1st. But, as it turned later, the stocks fell only on June 12th, which is 42 days later. What preposition should I use to state my error in days?:

1. I made a mistake in my predictions for 42 days.

or

1. I made a mistake in my predictions on 42 days.

or

1. I made a mistake in my predictions by 42 days.

or

1. I made a mistake in my predictions in 42 days.

or

1. I made a mistake in my predictions ______ 42 days. ?
• Your judgement turned out to be incorrect in 42 days. – Brandon Apr 3 at 9:41
• @Brandon - But does it not sound then as if my judgment was correct (or, perhaps, it was unknown whether it was correct or not) during those 42 days leading up to the 43rd day? – brilliant Apr 3 at 9:57
• A more idiomatic phrasing, at least in AmE, would be "My prediction was off by 42 days." or "My prediction was 42 days off." – Miles Apr 3 at 20:36
• @Miles - Can I do the same with the word "calculation" ("My calculation was off by 42 days")? – brilliant Apr 3 at 23:21
• Sure. Or more succinctly, if the context is clear, you could just say "I was off by 42 days." See also ell.stackexchange.com/questions/122726/… – Miles Apr 3 at 23:41

Only "by" would have the sense you want.

"for" 42 days means you made a mistake on each of 42 consecutive days. "on" would mean that out of the last X days, you made a mistake on 42 of them. "in" doesn't make sense at all.

• The most natural-sounding way to express this idea to me seems to be "My prediction was off by [72] days." (As I see Miles pointed out in a comment on the question.) – V2Blast Apr 3 at 22:58
• Would "in" have no sense at all? Two months ago I started my extensive stock analysis, and after 42 days of hard work, I produced a report that was all wrong. So I made my mistake in 42 days... – GSerg Apr 4 at 10:51
• I think the sentence sounds odd because it is incorrect. I made a mistake in my predictions means I made predictions (more than one) and made a mistake in those. But the mistake is being off by 42 days. The predictions are not mistaken by 42 days. – DonQuiKong Apr 4 at 15:59

The best preposition to use would be 'by'.

I made a mistake in my predictions by 42 days.

However, the sentence is still a bit awkward. It would be more idiomatic to say

My predictions were off by 42 days.

I would say "I made a mistake: my prediction was out by 42 days".

None of the above. The correct sentence is. "My prediction was off by 42 days."

I think it would be:

I made a mistake in my prediction of 42 days.

• @jpa sure, I encountered this answer in the review queue and didn't see the others. – Glorfindel Apr 5 at 16:42
• Absent context, this phrasing is ambiguous. It could be interpreted as OP intended, but it could also be misinterpreted that "of 42 days" is describing the prediction rather than the mistake. For example, OP predicted something would happen in 42 days, but that prediction ended up being incorrect. – Brian Apr 7 at 15:52