《英国医学会杂志》（BMJ）自2008年9月开始至2015年由两位流行病与统计学专家不间断地出了300多期statistical question系列。在这个系列中，两位学者每次出一道统计学选择题，进行选择并解释。现在我精选300道Statistical Question，形成中文版，请有兴趣的朋友们进行回答。
BMJ 统计问题（4）：下列哪些因素会影响临床试验样本量计算？（多选）
*请大家在看问题答案解析之前，先“投票”，也看看大家的结果
Which of the following will affect the sample size calculation for a clinical trial? (There may be more than one correct statement.)c) Size of effect to detectd) Variability of primary outcome
Answer：
These are the four pieces of information required for a sample size calculation.Larger sample sizes will result in increased power.Formal“significance” may be set at any P value. The smaller the P value chosen as indicating significance the larger will be the required sample size.Large clinical differences are easier to detect that small ones. The smaller the clinical effect that it is desired to detect, the larger will be the required sample size.If the outcome measure is highly variable this will act like “noise,” making it difficult to detect the “signal” of the clinical effect. Variability in a continuous measure may be assessed by the standard deviation. The larger this is, the larger will be the required sample size to detect a certain clinical effect. Trials are sometimes designed to reduce variability by using strictinclusion criteria to ensure that people in the trial are more similar (for example, similar stage of illness, similar age, similar ethnicity). This will reduce the required sample size at the expense of reducing wider generalisability.Power是检验效能。较大的样本量将导致检验效能增加。Significance level是检验水准。正式的“检验水准”可以设置为任何值。选择作为判断P值是否具有统计学意义的a值越小，所需的样本量就越大。Size of effect指的是效应值。较大的临床效果差异更容易发现较小的差异。预计检测到的临床效果越小，所需的样本量就越大。Variability是说临床结局的变异程度。如果结果变异程度很大，则它将像“噪声”一样起作用，从而使得难以检测到临床效果的“信号”。连续性定量数据的变异程度可以通过标准差来评估。它越大，检测到某种临床效果所需的样本量就越大。因此有时设计试验时，通过使用严格的纳入标准来减少变异性，以确保试验中的人更加相似（例如，相似的疾病阶段，相似的年龄，相似的种族）。这将减少所需的样本大小，但这以降低更广泛的通用性为代价。
