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Sample Size Determination Example

AREA: Means | Urology

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Allergan Neurogenic Detrusor Research

TitleEfficacy of Botulinum Toxin A Injection for Neurogenic Detrusor Overactivity and Urinary Incontinence:

A Randomized, Double-Blind Trial

Objective:  We determined the efficacy of onabotulinumtoxinA for neurogenic detrusor overactivity secondary to spinal cord injury or multiple sclerosis.

Year: 2011

Source: New England Journal of Medicine

Link: http://www.jurology.com/article/S0022-5347(11)00239-4/pdf

Clinical Area: Urology

Sample Size Section in Paper/Protocol:

“Sample size estimation was based on certain assumptions, including a week 6 mean change from baseline of 0.75 UI episodes, an SD of 0.85 UI episodes,α = 0.05 and β = 0.20. To detect a difference of 0.75 between treatment groups in the mean change from baseline in the number of UI episodes and assuming a 20% dropout rate it was necessary to enroll 56 patients, that is 28 per group. The calculation assumed a 2-sample procedure using 2-sided statistical testing.”


Note:
Final analysis was conducted using ANCOVA model using baseline as a covariate.
This study or resultant future studies could use effect of baseline covariate to adjust sample size downwards due to between-subjects variance decrease.

Summary of Necessary Parameter Estimates for Sample Size Calculation:

 Parameter Value
 Significance Level 0.05
 Expected Placebo Mean (from baseline) 5.6
 Expected Treatment Mean 4.85
 Standard Deviation (Common) 0.85
 Power 80%
Expected Dropout 20%

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Step 1:
Search for the Two Sample Student’s t-test (equal variances) from the Select Test Design & Goal window.

This can be done using the radio buttons or alternatively, you can use the search bar at the end of the Select Test Design & Goal window.

Sample Size Calculator Example- nQuery- Example 16- Img 01-Two Sample Student’s t-test (equal variances)

Step 2:
Enter the parameter values for sample size calculation taken from the study protocol.

Sample Size Calculator Example- nQuery- Example 16- img 02- Two Sample Student’s t-test (equal variances)

Step 3:
Once Power is entered, sample size is calculated automatically.

This gives a sample size of 22 per group


Sample Size Calculator Example- nQuery- Example 16- img 03- Two Sample Student’s t-test (equal variances)

Step 4:
Once the calculation is completed, nQuery Advanced provides an output statement summarizing the results. It States:

Output Statement:

A sample size of 22 in each group will have 80% power to detect a difference in means of 0.75 (the difference between a Group 1 mean, µ₁, of 5.6 and a Group 2 mean, µ₂, of 4.85) assuming that the common standard deviation is 0.85 using a two group t-test with a 5% two-sided significance level.

 

Sample Size Calculator Example- nQuery- Example 16- Img 04- Two Sample Student’s t-test (equal variances)

Step 4:
Calculate the sample size adjusted for expected 10% dropout, as outlined in protocol.

This gives a final sample size of 28 per group as per study design in protocol.


Note:
In the case of ANCOVA analysis, the only additional information necessary would be an estimate of R2 between the response and the baseline covariate (or average R2 if other covariates were used). For example, using the ANCOVA sample size table available in nQuery + nTerim 3.0 for the above study and assuming a 10% R2 between baseline and the response would have yielded an adjusted sample size of 25 per group. 

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