# P-value

can someone please tell me why when the p-value is close to zero the null is rejected??? i thought the p-value was the area of the two tails at the end of a two tail test… meaning… if the test statisitc fall within this range… the null is rejected?? I’m confused :s

Oops my mistake. I was thinking of the power of a test.

OK - This is wrong but the way to think about it for the test. The P-value is the probability that the null hypothesis is true. That means that low p-values cause you to reject the null hypothesis.

if your p-value is < level of significance of the test - you are in the tail area… so for a level of significance of 5% - any p-value < 5% will cause you to reject the Null hypothesis.

That’s what my ex-girlfriend said when she rejected me: “Your P-value is close to zero”.

It must be awful having a small “P-value”.

ok but if lets say you have a significance level of 5%… doesnt that mean that you have a 5% chance of rejecting the null? therefore a p-value of 0% means that u hvae a 0% chance of rejecting?

5% P-Value by definition means that there is only 5% chance that you will get that result in a completely random trial. So when you set your “significance level” to 5%, this means that you will assume that a result from the 5% tail is not random, i.e. it is “statistically significant”. So, if you P-Value is 0%, this means that there is a 0% chance that this result is completely random. In other words, there is a 100% chance that the result is explained by the alternate hypothesis. Therefore, you can reject the null hypothesis at any significance level. Does this make sense?

ooooooooooooooooooo!!! i was thinking p-value is a significance level. THANK YOU!

ohai Wrote: ------------------------------------------------------- > 5% P-Value by definition means that there is only > 5% chance that you will get that result in a > completely random trial. … if the null hypothesis is true >So when you set your > “significance level” to 5%, this means that you > will reject the null hypothesis because either a rare event has happened or the null hypothesis isn’t true. The P-value describes how rare the event is. For low P-values we choose “null hypothesis not true” instead of “rare event has happened” >assume that a result from the 5% tail is not > random, i.e. it is “statistically significant”. > > So, if you P-Value is 0%, this means that there is > a 0% chance that this result is completely random. And of course it most testing situations you can’t have a P-value of exactly 0 > In other words, there is a 100% chance that the > result is explained by the alternate hypothesis. > Therefore, you can reject the null hypothesis at > any significance level. > > Does this make sense?

So what type of question will I find in the exam about the P-Value? and how would I answer it???

1. A statistical test [about something] was conducted at significance level 0.05. The P-Value of the test was 0.08. Based on this P-Value, the experimenter can: a) Reject the alternative hypothesis and conclude that the null hypothesis is true b) Reject the null hypothesis and conclude that the alternative hypothesis is true c) Accept the alternative hypothesis and conclude it is true d) Fail to reject the null hypothesis and conclude that the test didn’t help in your understanding of the world and was, in fact, a major waste of time and company resources. 2) If you conduct a statistical test with a significance level of 0.05 but got a P-Value of 0.08 and then publish and distribute a research report saying that the test was significant at alpha = 0.1, according to the CFA ethical code this is: a) A violation of the principle 3 D: Misrepresentation b) A violation of principle 5 A: Reasonable basis only if the report contained a recommendation to buy or sell a security c) A violation of principle 5 A: Reasonable basis regardless of whether it contained a recommendation d) A violation of 4 A: Loyalty to employers because your test was a major waste of time and company resources. e) Not a violation Edit: Oh, I meant to add my signature

wow!!! thanks those questions sound hard…

1 D 2 A?

It’s definitely 1D. I have no idea about 2; it was just fun BS.