Saturday, December 18, 2010

Finding the p value in t-tests


The hypotheses that we have been working on for t-tests where there are two sample means drawn from two independent populations have all been two-tailed. It is perfectly possible to have a one-tailed test, but we haven’t learned that. So, when you look at the Excel output, you are interested only in the result for the two-tailed test. Excel provides a p value, as shown here.

t-Test: Paired Two Sample for Means




Current
Previous
Mean
0.5468
0.3404
Variance
0.305139
0.276987
Observations
25
25
Pearson Correlation
0.880067

Hypothesized Mean Difference
0

df
24

t Stat
3.889064

P(T<=t) one-tail
0.000349

t Critical one-tail
1.710882

P(T<=t) two-tail
0.000697

t Critical two-tail
2.063899


This output was found by using a paired t-test, using the Earnings2005 data (you have the file). Because the p value for two-tailed is 0.000697, we reject the null hypothesis: p is smaller than 0.05. If we reject the null, we are saying that the two population means are not the same. So look at the top line of means, and you can see that ‘current’ is larger than ‘previous’.

4 comments:

Anonymous said...

I all the time emailed this weblog post page to all my associates, as if like to read it afterward my links will too.
Feel free to visit my webpage ; benefits of hcg diet for men

Anonymous said...

I rarely leave a response, but after browsing a
bunch of responses on "Finding the p value in t-tests".
I actually do have a couple of questions for you if you do not mind.
Is it just me or does it look as if like a few of these responses
come across as if they are coming from brain dead people?
:-P And, if you are writing on other social sites, I'd like to follow you. Could you make a list of every one of your social community sites like your twitter feed, Facebook page or linkedin profile?
my website - New Bubble Shooter

Anonymous said...

Schooling and indoctrination are often confused. The standard text guide reply,?
may possibly actually be erroneous? but a pupil who can begin to see the error clearly,
is marked down being a 'heretic', but a pupil who learns the official appropriate
answer just like a trained parrot, passes the course!
Who determines the 'answer'? Hundreds of billions
of dollars in corporate profit might be at stake, as together with the erooneous hypothesis of 'chemical inbalance' within the brain
requiring bogus pharmeceutical intervention? so CAVEAT!


Feel free to visit my web site Premature ejaculation Propecia

Anonymous said...

lol that was wonderful. Lesbosexual? xD I realize this is planning to seem so random, but exactly where did Bill get his sweater?
(he's the one wearing the Harry Potter glasses correct?) Just? randomly wondering due to the fact I just got a hat that matches it perfectly lol.

Also visit my homepage - marcus london 60 minute stamina for free