Smith Blair H, Hannaford Philip C, Elliott Alison M, Smith W Cairns, Chambers W Alastair
Department of General Practice and Primary Care, University of Aberdeen, UK.
Fam Pract. 2005 Apr;22(2):205-14. doi: 10.1093/fampra/cmi009. Epub 2005 Feb 18.
Sampling for primary care research must strike a balance between efficiency and external validity. For most conditions, even a large population sample will yield a small number of cases, yet other sampling techniques risk problems with extrapolation of findings.
To compare the efficiency and external validity of two sampling methods for both an intervention study and epidemiological research in primary care--a convenience sample and a general population sample--comparing the response and follow-up rates, the demographic and clinical characteristics of each sample, and calculating the 'number needed to sample' (NNS) for a hypothetical randomized controlled trial.
In 1996, we selected two random samples of adults from 29 general practices in Grampian, for an epidemiological study of chronic pain. One sample of 4175 was identified by an electronic questionnaire that listed patients receiving regular analgesic prescriptions--the 'repeat prescription sample'. The other sample of 5036 was identified from all patients on practice lists--the 'general population sample'. Questionnaires, including demographic, pain and general health measures, were sent to all. A similar follow-up questionnaire was sent in 2000 to all those agreeing to participate in further research. We identified a potential group of subjects for a hypothetical trial in primary care based on a recently published trial (those aged 25-64, with severe chronic back pain, willing to participate in further research).
The repeat prescription sample produced better response rates than the general sample overall (86% compared with 82%, P < 0.001), from both genders and from the oldest and youngest age groups. The NNS using convenience sampling was 10 for each member of the final potential trial sample, compared with 55 using general population sampling. There were important differences between the samples in age, marital and employment status, social class and educational level. However, among the potential trial sample, there were no demographic differences. Those from the repeat prescription sample had poorer indices than the general population sample in all pain and health measures.
The repeat prescription sampling method was approximately five times more efficient than the general population method. However demographic and clinical differences in the repeat prescription sample might hamper extrapolation of findings to the general population, particularly in an epidemiological study, and demonstrate that simple comparison with age and gender of the target population is insufficient.
初级保健研究中的抽样必须在效率和外部效度之间取得平衡。对于大多数疾病，即使是大量的人群样本也只会产生少量病例，而其他抽样技术则存在研究结果外推的问题。
比较两种抽样方法在初级保健干预研究和流行病学研究中的效率和外部效度，即便利样本和一般人群样本，比较其应答率和随访率、每个样本的人口统计学和临床特征，并计算假设随机对照试验的“抽样所需数量”（NNS）。
1996年，我们从格兰扁地区的29家全科诊所中选取了两个成年人随机样本，用于慢性疼痛的流行病学研究。一个由4175人组成的样本通过电子问卷确定，问卷列出了接受定期镇痛处方的患者——“重复处方样本”。另一个由5036人组成的样本从诊所名单上的所有患者中确定——“一般人群样本”。向所有人发送了包括人口统计学、疼痛和一般健康指标的问卷。2000年，向所有同意参与进一步研究的人发送了类似的随访问卷。我们根据最近发表的一项试验（年龄在25 - 64岁、患有严重慢性背痛、愿意参与进一步研究的人）确定了一组初级保健中假设试验的潜在受试者群体。
总体而言，重复处方样本的应答率高于一般样本（分别为86%和82%，P < 0.001），男性和女性以及年龄最大和最小的年龄组均如此。使用便利抽样时，最终潜在试验样本的每个成员的NNS为10，而使用一般人群抽样时为55。样本在年龄、婚姻和就业状况、社会阶层和教育水平方面存在重要差异。然而，在潜在试验样本中，不存在人口统计学差异。在所有疼痛和健康指标方面，来自重复处方样本的人的指标比一般人群样本差。
重复处方抽样方法的效率约为一般人群抽样方法的五倍。然而，重复处方样本中的人口统计学和临床差异可能会妨碍将研究结果外推至一般人群，尤其是在流行病学研究中，这表明仅与目标人群的年龄和性别进行简单比较是不够的。
