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P Values - part 3
The P value as a
‘statistic’
Robin Beaumont
1/03/2012
With much help from
Professor Geoff Cumming
P values - Putting it all together
Summary
Reviewso far
• A P value is a conditional probability which
considers a range of outcomes – shown as a
‘area’ in a graph.
• The SEM formula allows us to: predict the
accuracy of your estimate ( i.e. the mean
value of our sample) across a infinite
number of samples!
Summary
far
What is a so
statistic?
• A statistic is just a summary measure,
technically we have reduced a set of data to
one or two values:
• Range (smallest – largest)
• Mean, median etc.
• Inter-quartile range, SD Variance
• Z score, T value, chi square value, F value etc
• P value
T value
• T statistic – different types, simplest 1 sample:
Tstatistic  observed
difference in estimated mean and population value
sampling variability in means
Tstatistic  observed
difference in estimated mean and population value
SEM

observed difference in estimated mean and population value
expected variability in means due to random samping
Signal

Noise
So when t = 0 means 0/anything = estimated and
hypothesised population mean are equal
So when t = 1 observed different same as SEM
So when t = 10 observed different much greater than
SEM
T statistic example
Serum amylase values from a random sample of 15 apparently healthy
subjects. The mean = 96 SD= 35 units/100 ml.
How likely would such a ‘unusual’ sample be obtained from a
population of serum amylase determinations with a mean of 120.
(taken from Daniel 1991 p.202 adapted)
The population value = the null
hypothesis
Tstatistic 
96  120
24


35
9.037
15
2.656
This looks like a rare occurrence?
t density:
s x = 9.037 n =15
120
96
Given that the sample was obtained
from a population with a mean of 120
a sample with a T(n=15) statistic of 2.656 or 2.656 or one more extreme
will occur 1.8% of the time = just
under two samples per hundred on
Shaded
area
average.
=0.0188
....
What does the shaded
Given that the sample was obtained
area mean!
from a population with a mean of 120
0
a sample of 15 producing a mean of 96
2.656
0
-2.656
t
(120-x where x=24) or 144 (120+x
Serum amylase values from a random sample of 15 apparently
healthy subjects. mean =96 SD= 35 units/100 ml.
where x=24) or one more extreme will
How likely would such a unusual sample be obtained from a
population of serum amylase determinations with a mean of 120.
occur 1.8% of the time, that is just
(taken from Daniel 1991 p.202 adapted)
under two samples per hundred on
=P value
average.
P value = 2 · P(t < t| H is true) = 2 · [area to the left of t under a t distribution with df = n − 1]
Original units:
(n−1)
o
P value and probability for the one sample t statistic
p value
= 2 x P(t(n-1) values more extreme than obtained t(n-1) | Ho is true)
= 2 X [area to the left of t under a t distribution with n − 1 shape]
Statistic -> sampling distribution -> PDF -> p value
No sampling distribution! Create a virtual one
P Value Variability
Taking another random sample the P value be different
How different? – Does not follow a normal distribution
Depends upon the probability of the null hypothesis
being true! Remember we have assumed so far that the
null hypothesis is true.
Dance of the p values – Geoff Cummings
Simplified dance of the p values when the null hypothesis is true
Example from Geoff
Cummings dance of the p
values
The take home message is
that we can obtain very small
p values even when the null
hypothesis is true.
Why no CI for the P Value if it varies across trials
• P value -> statistic but
• Not all statistics represent values that are reflected in a
population value
• Other ways of getting an idea of variability across trials:
• Reproducibility Probability Value (RP)
Goodman 1992 and also 2001 journal articles
Hung, O’Neill, Bauer & Kohne 1997 Biometrics journal
Shao & Chow 2002 – Statistics in Medicine journal
Boos & Stefanki 2011 – Journal of the American statistical
association
Cummings 2008 + and book
Cumming’s Reproducibility (replication) Probability Value
Given Pobtained = 0.05
What is the interval in which
we are likely to see 80% of
subsequent P values?
Answer:
We have 80% of seeing
subsequent p values fall within
the zero to 0.22 boundary
0, 0.22 [One sided]
This means that we have a
20% of them being
subsequently > 0.22
What about when the null hypothesis is not true?
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