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Sample Size Determination Example

AREA: Means | Nephrology

nQuery In Use Background Web  Opacity 35

Baxter Nephrology Research 

Title"Flush before fill" in children receiving automated peritoneal dialysis

Objective: To evaluate the impact of the "flush before fill" technique on the frequency of peritonitis in children receiving automated peritoneal dialysis (APD). DESIGN: Randomized prospective multicenter study. SETTING: Participating pediatric dialysis programs of the Pediatric Peritoneal Dialysis Study Consortium.

Year: 2003

Source: Peritoneal Dialysis International

Link: http://www.pdiconnect.com/content/23/5/493.short

Clinical Area: Nephrology

Sample Size Section in Paper/Protocol:

“Calculation of the required sample size was based on the assumption that the introduction of the flush-before-fill technique would reduce the peritonitis rate from a baseline study value of 1 infection every 12 patient months to 1 infection every 24 patient months. With a two-sided test, an alpha level of 0.05, and a power of 80%, the analysis required 48 patients per group and 566 patient months per group”

 

Note: Rates are converted to years rates due to study being for 1 year.


Summary of Necessary Parameter Estimates for Sample Size Calculation:

 Parameter Value
 Significance Level (2-Sided) 0.05
 Expected Placebo Rate 1
 Expected Treatment Mean 0.5
 Power 80%

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Step 1: 
Select the Two Group Test of Incidence Rates using Normal Approximation table 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 09- Img 01- Two Group Test of Incidence Rates using Normal Approximation

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

Sample Size Calculator Example- nQuery- Example 09- Img 02- Two Group Test of Incidence Rates using Normal Approximation

Step 3:
Select “Sample Size Calculation” and Click Run.

This gives a sample size of 48 per group (since study was one year long) with power of 80.7%

 

Sample Size Calculator Example- nQuery- Example 09- Img 03- Two Group Test of Incidence Rates using Normal Approximation

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

Output Statement:

A 2 sided comparison of two incidence rates in terms of person years in a fully randomized trial with a sample of 48 person-years per group would achieve 80.74% power at the 0.05 significance level to detect a difference in incidence rates of at least 0.5 if the control group incidence rate was 1.

 

Sample Size Calculator Example- nQuery- Example 09- Img 04- Two Group Test of Incidence Rates using Normal Approximation

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