WBR0435

Author	[[PageAuthor::Yazan Daaboul, M.D. (Reviewed by Alison Leibowitz) (Reviewed by Serge Korjian)]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Biostatistics/Epidemiology
Sub Category	SubCategory::Renal
Prompt	[[Prompt::A researcher conducts a cross-sectional study to identify factors that are associated with hemodialysis adequacy among 90 patients undergoing hemodialysis. Following data collection, statistical analysis reveals that the urea reduction rate (URR) is associated with hemodialysis adequacy with a p-value of 0.12 (significant p-value <0.05). Which of the following modifications will most likely increase the statistical power of the study?]]
Answer A	AnswerA::Recruiting more hemodialysis patients to the study
Answer A Explanation	AnswerAExp::Recruiting more patients, who fit the inclusion criteria, can increase the statistical power of the study.
Answer B	AnswerB::Comparing peritoneal dialysis patients to hemodialysis patients
Answer B Explanation	AnswerBExp::Since the study does not involve peritoneal dialysis patients, adding those to the study is not appropriate and will most likely limit the study’s statistical power.
Answer C	[[AnswerC::Decreasing the p-value cut-off to <0.01]]
Answer C Explanation	[[AnswerCExp::α (type 1 error) represents the likelihood of obtaining a significant difference when there actually is none. The p-value represents the α or type 1 error of a study. As a rule, for the same population size, as α decreases (p-value cut-off becomes more stringent), β increases. As power = (1-β), higher β values lower the power of a study. Therefore decreasing the p-value cut-off to <0.01 will decrease the statistical power of the study.]]
Answer D	AnswerD::Repeating all measurements of urea reduction rate (URR) for the 90 patients who are recruited
Answer D Explanation	AnswerDExp::Repeating measurements will test the precision of the study’s measurements. Since measurements were described as precise in the first place, the statistical power of the study is likely not to be affected.
Answer E	AnswerE::The statistical power of the study cannot be changed
Answer E Explanation	AnswerEExp::The statistical power of the study can be changed if the sample size is increased.
Right Answer	RightAnswer::A
Explanation	[[Explanation::The statistical power of a study is defined as: Power = (1-β), where β is the type II error (perceiving there is no effect when there actually is). The power of the study is dependent on the sample size. Increasing the number of hemodialysis patients enrolled in the study will increase the power. Educational Objective: Statistical power of a study is defined as: Power = (1-β), where β is the type II error. The power increases when the study sample size increases, or when the significance level is lowered. References: Chow S, Liu J. Design and Analysis of Clinical Trials, Concepts and Methodologies. John Wiley & Sons; 2013. First Aid 2014 page 57]]
Approved	Approved::Yes
Keyword	WBRKeyword::Urea reduction rate, WBRKeyword::URR, WBRKeyword::Hemodialysis, WBRKeyword::Statistical power, WBRKeyword::Power, WBRKeyword::Type II, WBRKeyword::Error, WBRKeyword::Sample size, WBRKeyword::Confidence, WBRKeyword::Interval, WBRKeyword::Confidence interval
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