Odd Z table value

[Joli]

Hey everyone,

I'm trying to make sense of a Z value I wrote down in class.

I understand how the Z table works now, but before I got there I made a note in class about a certain Z value and looking through the table I can't make sense of the note now. Perhaps I wrote it down wrong. Any ideas?

I wrote: Z0.2 = 0.084

Can you perhaps make sense out of it?

Thanks in advance,
Joli

 

[Dason]

Well the .2 percentile is about -.84 so that's probably related. Maybe you accidentally added a 0 in there?

 

[EdGr]

Not sure what Dason means by the 0.2 percentile: that would be a fraction of 1%. I'm guessing his or her meaning is the 20th percentile, which is probably right because this is a very common cutoff for power calculations. Is it possible that was the context? 20% of z values are greater than 0.841 (and 80% are greater than -.841) so formulas for 80% power often use z for beta = 0.2 as -0.841.

 

[Joli]

Yes it turns out that I did add the 0 by accident and the negative number also had me confused but of course the curve is symmetrical, silly me. So yes, it was about a right sided p-value of 0.2, the outcome of which is 0.84. Well at least this conundrum has helped me understand the Z-table better. Thanks for answering!