# Help with P-Value

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• Dec 6th 2013, 10:21 AM
arcticreaver
Help with P-Value
So i get a problem where I must find the p-value.

I am given the following information:

x=21
n=25
standard deviation is 5

H0: u=20
H1: u not equal to 20

so i'm confused. since it does not give me a t value to base this stuff off of like .5 or .025, etc.

don't i need that to solve this problem?
• Dec 6th 2013, 11:01 AM
romsek
Re: Help with P-Value
P-Value is the probability of your x given the Null hypotheses (H0) is true.

Here your H0 is Normal(20,5/sqrt(25)) (do you understand why?) = Normal(20,1)

Normalize x to obtain ((x - mean)/stdev), in this case ((21 - 20)/1) = 1

and use your tables or whatever to find the Probability of observing 1 under the Normal(0,1) distribution.

I get 0.841
• Dec 6th 2013, 11:27 AM
arcticreaver
Re: Help with P-Value
right 1 is 0.841

so now i need to subtract 1 - .841, right? with is .159, but this is not the correct p-value, since the answer it's giving me is like .3174...
• Dec 6th 2013, 11:40 AM
romsek
Re: Help with P-Value
Quote:

Originally Posted by arcticreaver
right 1 is 0.841

so now i need to subtract 1 - .841, right? with is .159, but this is not the correct p-value, since the answer it's giving me is like .3174...

0.841 is your p-value for your observations x=21
• Dec 6th 2013, 11:56 AM
arcticreaver
Re: Help with P-Value
well that is not what the answer key is saying.

it says the P-Value is .3174
• Dec 6th 2013, 12:35 PM
romsek
Re: Help with P-Value
Quote:

Originally Posted by arcticreaver
well that is not what the answer key is saying.

it says the P-Value is .3174

Ok, I misinterpreted what I was reading. Because of the way this problem is set up as the hypotheses being u=20 and u <> 20

You need to take the sum of the 2 two tails i.e. 2 (1 - Pr[x])

Here is an example of the type of problem you are doing with an explanation.
• Dec 6th 2013, 12:58 PM
romsek
Re: Help with P-Value
Quote:

Originally Posted by romsek
Ok, I misinterpreted what I was reading. Because of the way this problem is set up as the hypotheses being u=20 and u <> 20

You need to take the sum of the 2 two tails i.e. 2 (1 - Pr[x])

Here is an example of the type of problem you are doing with an explanation.

This is a better explanation. Basically because this test is a u=20 vs. u<>20 test you need to use a 2 tailed test.
• Dec 6th 2013, 02:24 PM
arcticreaver
Re: Help with P-Value
omg, thanks for that LINK!!!! i get it now!