biostatistics case studies 2010
DESCRIPTION
Biostatistics Case Studies 2010. Session 1: Study Size Tutorial. Peter D. Christenson Biostatistician http://gcrc.labiomed.org/biostat. First, Use software for three papers. Then, Discuss some logic. Paper #1. How was N=498 determined?. - PowerPoint PPT PresentationTRANSCRIPT
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Biostatistics Case Studies 2010
Peter D. Christenson
Biostatistician
http://gcrc.labiomed.org/biostat
Session 1:
Study Size Tutorial
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First, Use software for three papers.
Then, Discuss some logic.
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Paper #1
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How was N=498 determined?
What reduction in CVD events can 224 + 224 subjects detect? Nevertheless
How many subjects would be needed to detect this Δ?
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Software Output for % of CVD Events
224 + 224 → detect 6.7% vs. 1.13%, i.e., 88% ↓.
Need 3115 + 3115 to detect 25% ↓ from 6.7% to 5%, i.e., a total of (3115+3115)/0.9 = 6922.
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From earlier design paper (Russell 2007):
Δ = 0.85(0.05)
mm = 0.0425 mm
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Software Output for Mean IMT
Each group N for 10% Dropout → 0.9N = 224
→ N = 224/0.9 = 249. Total study size = 2(249)=498
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Paper #2
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Williamson paper
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Software Output - Percentages
Slightly larger Ns due to slightly different test to be used.
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Software Output - Means
Can detect 0.4 SDs. Units? Since normal range =~ 6SD, this corresponds to ~0.4/6=7% shift in normal range.
Applies to any continuously measured outcome.
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Paper #3
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From Nance paper
Δ = ~8%
Δ
SD√(1/N1 + 1/N2)= 2.82
Solve for SD to get SD =~ 6.8%
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Software Output for Gilchrist Paper
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Some Logic
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How was 498 determined?
Back to:
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How IMT Change Comparison Will be Made
Strength of Treatment Effect:
Signal:Noise Ratio t=
Observed Δ
SD√(1/N1 + 1/N2)
Δ = Aggressive - Standard Mean Diff in IMT changes
SD = Std Dev of within group IMT changes
N1 = N2 = Group size
| t | > ~1.96 ↔ p<0.05
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Could Solve for N
Observed Δ
SD√(1/N1 + 1/N2)
This is not quite right.
The Δ is the actual observed difference.
This sample Δ will vary from the real Δ in “everyone”.
Need to increase N in case the sample happens to have a Δ that is lower than the real Δ (50% possibility).
≥~1.96 if (with N = N1 = N2):
Δ ≥ 1.96SD√(2/N) or N ≥ 2SD2
Δ2
(1.96)2
t =
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Need to Increase N for Power
Need to increase N to:
2SD2
Δ2
(1.96 + 0.842)2
Power is the probability that p<0.05 if Δ is the real effect, incorporating the possibility that the Δ in our sample could be smaller.
2SD2
Δ2
(1.96)2N = for 50% power.
for 80% power.N =
N =2SD2
Δ2(1.96 + 1.282)2 for 90% power.
from Normal Tables
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Info Needed for Study Size: Comparing Means
1. Effect
2. Subject variability
3. p-value (1.96 for p=0.05; 2.58 for p=0.01)
4. Power (0.842 for 80% power; 1.645 for 95% power)
(1.96 + 0.842)22SD2
Δ2
N =
Same four quantities, but different formula, if comparing %s, hazard ratios, odds ratios, etc.
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(1.96 + 0.842)2 2(0.16)2
(0.0425)2N = = 224
Each group N for 10% Dropout → 0.9N = 224
→ N = 224/0.9 = 249. Total study size = 2(249)=498
2SD2
Δ2
N = (1.96 + 0.842)2
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Change Effect Size to be Detected
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SD Estimate Could be Wrong
Should examine SD as study progresses.
May need to increase N if SD was underestimated.
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Recall: Software Output for % of CVD Events
224 + 224 → detect 6.7% vs. 1.13%, i.e., 88% ↓.
Need 3115 + 3115 to detect 25% ↓ from 6.7% to 5%, i.e., a total of (3115+3115)/0.9 = 6922.
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Comparing Survival
We just saw that:
Need 3115 + 3115 to detect 25% ↓ from 6.7% to 5%, i.e., a total of (3115+3115)/0.9 = 6922.
This does not use the 10% of 6922 = 692 subjects lost to follow-up (in the analysis).
Recall that survival analysis, e.g., Kaplan-Meier curves, does use the info from these 692 subjects for as long as they were observed.
So, fewer than 6922 subjects are needed using survival analysis - we calculate that N now.
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Comparing Survival
Really 0.
This software requires
>0.
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Comparing Survival
Total of 6304, includes 10% loss, compared to 6922.
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Some Study Size Software
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Free Study Size Software
www.stat.uiowa.edu/~rlenth/Power
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Study Size Software in GCRC Lab
ncss.com ~$500
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nQuery - Used by Most Drug Companies