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S2 - Hypothesis testing???

Let's say that the upper critical value for a two-tailed region with 0.05 significance is x.

P(X ≥ x) ≤ 0.05
P(X ≤ x - 1) ≥ 0.95

x is the value that I'd expect to be for the critical value (and what I've been taught), but sometimes it's x - 1 on some mark schemes and it's really confusing me, and I don't know which one to use or if I'm doing it right.

Please help

Reply 1

sleepyspider

bump, i'm desperate D:

Reply 2

rich1334

I did S2 OCR this year and only ever came across the first method. That's what I'd stick to. Also for a two tailed test @ 5% sig level, each tail is 2.5% :smile:

hope that helps!

Reply 3

sleepyspider

Original post by rich1334

I did S2 OCR this year and only ever came across the first method. That's what I'd stick to. Also for a two tailed test @ 5% sig level, each tail is 2.5% :smile:

hope that helps!

It did, thank you!

It's just the mark schemes that are confusing me; I've always been taught, and every single revision guide tells me to look at the value in the row below the row containing the probability you're looking at (if that makes any sense), but some mark schemes don't have that as the answer, and it confuses me. :/

This picture hopefully explains what I'm trying to say. This is what I've been taught to do (the critical values would be 3 and 18) but some mark schemes would have the upper critical value as 17, the row in which the circled probability is in.