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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% |

**Step 1:**Search for the

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.

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

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

This gives a sample size of 22 per group |

**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. |

**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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