mps/msc in statisticsadaptive & bayesian - lect 41 lecture 4 sample size reviews 4.1a general...
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MPS/MSc in Statistics Adaptive & Bayesian - Lect 4 1
Lecture 4
Sample size reviews
4.1 A general approach to sample size reviews
4.2 Binary data
4.3 Normally distributed data
4.4 Exact results: normally distributed responses
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MPS/MSc in Statistics Adaptive & Bayesian - Lect 4 2
4.1 A general approach to sample size reviews
• Many sample size formulae depend on nuisance parameters, the values of which have to be guessed
• Part way through the trial we will have plenty of data on which to base a better guess
• So, do that, and recalculate the sample size• Now use the new sample size, perhaps within the limits of
minimum and maximum possible values• Assess the effect of this procedure on type I error: usually
it is very small
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MPS/MSc in Statistics Adaptive & Bayesian - Lect 4 3
This is an adaptive design with a single interim analysiswhich may lead only to a reassessment of sample size
Its use is becoming widespread, and the regulatory authoritiesare generally well-disposed towards it
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MPS/MSc in Statistics Adaptive & Bayesian - Lect 4 4
4.2 Sample size review for binary data
Treatments: Experimental (E) and Control (C)
Success probabilities: pE and pC
Hypotheses: H0: pE = pC H1: pE > pC
Type I error: a (one-sided)
Power: 1 – b, when pE = pER and pC = pCR
Sample sizes: nE and nC, where nE + nC = n
Allocation ratio: (1:1), that is nE = nC
pCR is the anticipated value of pC, and an improvement from
that value to pE = pER on E would be clinically worthwhile
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MPS/MSc in Statistics Adaptive & Bayesian - Lect 4 5
Probability difference approach
Initial sample size calculation
Put q = pE - pC, and set power at q = qR = pER - pCR
Two popular formulae for n are:
(Machin et al., 1997)
and
(4.2) 2
1 1 1ER CR2
R
z zn 4p(1 p) phe pw re p
2
1 1 ER ER CR CR
R
z 2p(1 p) z p (1(4.1
p ) p (1 p )n )2
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MPS/MSc in Statistics Adaptive & Bayesian - Lect 4 6
To use these formulae:
• use previous data and experience to guess pCR
• consider what difference qR would be clinically important
• deduce pER and
Using these values, find the required sample size n
Then, when data from about patients are available, a sample size review can be conducted
p
12 n
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MPS/MSc in Statistics Adaptive & Bayesian - Lect 4 7
At the sample size review we do not change qR:
- this remains the clinically important difference
To recompute n based on (4.1), identify the control patients andfind an estimate, of pC as the success rate on C so far
Replace pCR by , pER by and
To recompute n based on (4.2), we need not break the blinding:just estimate as the overall success rate in the trial as a whole(over E and C)
The preservation of blindness makes the second option moreattractive
1C R2
ˆp by p
Cp̂
Cp̂ C Rp̂
p
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MPS/MSc in Statistics Adaptive & Bayesian - Lect 4 8
Log-odds ratio approach
Initial sample size calculation
Put
and set power at q = qR computed from the values pER and pCR The resulting sample size formula is:
(4.3)
This formula can be updated at a sample size review in thesame way as (4.2), without breaking the blind
2
1 1
R
z z4n
p(1 p)
E C CE
C E E C
p 1 p pplog log log
p 1 p 1 p 1 p
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MPS/MSc in Statistics Adaptive & Bayesian - Lect 4 9
Example
a = 0.025, 1 – b = 0.90: z1-a = 1.96, z1-b = 1.282
pCR = 0.3, pER = 0.5, = 0.4
(4.1) prob diff: qR = 0.2 n = 248
(4.2) prob diff: qR = 0.2 n = 252
(4.3) log-odds ratio: qR = 0.847 n = 244
p
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MPS/MSc in Statistics Adaptive & Bayesian - Lect 4 10
After 120 observations, the sample size is reviewed
We find that = 0.2, rather than 0.4
Retaining probability difference: qR = 0.2, (4.2) n = 168
- sample size goes down
- qR = 0.2 consistent with pCR = 0.1, pER = 0.3
Retaining log-odds ratio: qR = 0.847, (4.3) n = 366
- sample size goes up
- qR = 0.847 consistent with pCR = 0.134, pER = 0.266
p
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MPS/MSc in Statistics Adaptive & Bayesian - Lect 4 11
4.3 Normally distributed data
Treatments: Experimental (E) and Control (C)
Distributions: N(mE, s2) and N(mC, s2)
Hypotheses: H0: mE = mC H1: mE > mC
Type I error: a (one-sided)
Power: 1 – b, when mE = mER and mC = mCR
Sample sizes: nE and nC, where nE + nC = n
Allocation ratio: (1:1), that is nE = nC
Put q = mE - mC and qR = mER - mCR
Let denote the anticipated common variance
2R
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MPS/MSc in Statistics Adaptive & Bayesian - Lect 4 12
The sample size is given by
see Slide 2.8
The actual values of mER and mCR have no effect on n otherthan through qR
The anticipated variance is very influential, and is replacedby an estimate at the sample size review
2
1 12R
R
z zn 4 ( 4. ) 4
2R
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MPS/MSc in Statistics Adaptive & Bayesian - Lect 4 13
Estimating s2
(a) Use the conventional unbiased estimate
based on the n observations available so far
To use this requires breaking the blind, at least as far as
separating the two treatment groups: their identities need not
be revealed
22
hE E hC C2 2 x x x xˆ S
n 2
(4.5)
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14MPS/MSc in Statistics Adaptive & Bayesian - Lect 4
(b) Avoid unblinding, using a simple adjustment (Gould, 1995)
For each term in (4.5)
Substitute in (4.5)
E E E
E
n n n2 2 2 2
hE E hE E hE E Eh 1 h 1 h 1
2n2 C
hE E C Eh 1
x x x x x x (x x) n (x x )
n(x x) n (x x )
n
22 2 2C EhE E hC C hj C E
j E,C h
n nx x x x (x x) (x x )
n
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MPS/MSc in StatisticsAdaptive & Bayesian - Lect 4 15
If desired difference is present:
Can use estimate for sample size review without unblinding
(4.6)
: The estimate of total variance
E C Rx x
jCE
nnn22 2 2E C
hE E hC C hj Rh 1 h 1 j E,C h 1
n nx x x x (x x)
n
2%
jn
2 2E C 2 2E Chj R T Rj E,C h 12
n n n n(x x) n 1n nn 2 n 2
%%
2T
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MPS/MSc in Statistics Adaptive & Bayesian - Lect 4 16
(c) Avoid unblinding, using an E-M algorithm (Gould and Shih, 1992; Gould, 1995)
DO NOT USE THIS METHOD!
See Friede and Kieser (2002) to see why you should not
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MPS/MSc in Statistics Adaptive & Bayesian - Lect 4 17
Example
a = 0.025, 1 – b = 0.90: z1-a = 1.96, z1-b = 1.282
qR = 0.5, sR = 1.0
(4.4) n = 168
After 80 patients:
E E E C C Cn 40, x 5.6, S 1.45 n 40, x 5.3, S 1.26
2T 1.844
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MPS/MSc in Statistics Adaptive & Bayesian - Lect 4 18
Unblinded approach
From (4.5)
So that
From (4.4), new sample size is
2 2 2E E C Cˆ(n 2) (n 1) S (n 1) S
143.9139
2ˆ 1.845
n 310
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MPS/MSc in Statistics Adaptive & Bayesian - Lect 4 19
Blinded approach
From (4.6)
From (4.4), new sample size is
2
2
80145.7139 0.5
478
145.7139 5
781.804
n 304
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MPS/MSc in Statistics Adaptive & Bayesian - Lect 4 20
Consider the unblinded approach to sample size review
An initial sample size is set
When n1 = patients have given responses, s2 isestimated by the usual pooled variance
The sample size is then recalculated (as n2) and that numberof subjects is taken, provided that n2 ≥ n1
Finally, a t-test is performed
4.4 Exact results: normally distributed responses
12 n
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MPS/MSc in Statistics Adaptive & Bayesian - Lect 4 21
The final t-statistic is
where the subscript 2 identifies values computed for the finalsample
Now
E2 C22
2
x xt n
2S
2 22 E2 E2 C2 C22
2
(n 1)S (n 1)SS
n 2
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MPS/MSc in Statistics Adaptive & Bayesian - Lect 4 22
With the subscript + relating to the extra patients recruited afterthe sample size review, it can be shown that
Dividing by the true value of s2, the quantities on the rhs aredistributed as independent c2 random variable on nE1 – 1, nE+ - 1 and 1 degrees of freedom respectively
A similar result holds for the control group
2E2 E2
22 2 E1 EE1 E1 E E E1 E
E2
n 1 S
n n n 1 S n 1 S x x
n
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MPS/MSc in Statistics Adaptive & Bayesian - Lect 4 23
The final t-statistic, is a function of
and these quantities are mutually independent
From this result, the conditional distribution of t, given ,can be deduced
The revised sample size depends on the data only through
2 21E 1C E C 1x ,x ,x ,x an,S Sd
21S
21S
22 2 1n n S
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MPS/MSc in Statistics Adaptive & Bayesian - Lect 4 24
The density of t is given by
where W represents the random variable
is a t density with degrees of freedom specified by
: it is a step function in w
is a c2 density with n1 - 2 degrees of freedom
In this way, for a given value of s2, the exact properties of thesample size review procedure can be determined
T WT Wf (t) f (t w)f (w)dw
2 21 1n 2 S /
21S
T Wf (t w)
Wf (w)
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MPS/MSc in Statistics Adaptive & Bayesian - Lect 4 25
Example
a = 0.05, 1 – b = 0.90, = 1, qR = 0.414 n = 200
A sample size review is conducted after 100 responses
Suppose that the true value of s2 is 1.43
Then the true type I error rate will be 0.051
The true power will be 0.920
2R
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MPS/MSc in Statistics Adaptive & Bayesian - Lect 4 26
References
General methods for sample size review:
Wittes and Brittain (1990)Gould (1992)Birkett and Day (1994)
Exact evaluations:
Kieser and Friede (2000)Friede and Kieser (2006)
Montague (2007)