Department of Biostatistics and Medical Informatics, University College of Medical Sciences, New Delhi - 110 095, India.
Indian J Ophthalmol. 2010 Nov-Dec;58(6):519-22. doi: 10.4103/0301-4738.71699.
Sensitivity and specificity measure inherent validity of a diagnostic test against a gold standard. Researchers develop new diagnostic methods to reduce the cost, risk, invasiveness, and time. Adequate sample size is a must to precisely estimate the validity of a diagnostic test. In practice, researchers generally decide about the sample size arbitrarily either at their convenience, or from the previous literature. We have devised a simple nomogram that yields statistically valid sample size for anticipated sensitivity or anticipated specificity. MS Excel version 2007 was used to derive the values required to plot the nomogram using varying absolute precision, known prevalence of disease, and 95% confidence level using the formula already available in the literature. The nomogram plot was obtained by suitably arranging the lines and distances to conform to this formula. This nomogram could be easily used to determine the sample size for estimating the sensitivity or specificity of a diagnostic test with required precision and 95% confidence level. Sample size at 90% and 99% confidence level, respectively, can also be obtained by just multiplying 0.70 and 1.75 with the number obtained for the 95% confidence level. A nomogram instantly provides the required number of subjects by just moving the ruler and can be repeatedly used without redoing the calculations. This can also be applied for reverse calculations. This nomogram is not applicable for testing of the hypothesis set-up and is applicable only when both diagnostic test and gold standard results have a dichotomous category.
灵敏度和特异性衡量诊断测试相对于金标准的固有有效性。研究人员开发新的诊断方法以降低成本、风险、侵袭性和时间。足够的样本量是准确估计诊断测试有效性的必要条件。在实践中，研究人员通常根据自己的方便或从以前的文献中任意决定样本量。我们设计了一个简单的诺模图，可以为预期的灵敏度或特异性提供具有统计学意义的有效样本量。使用 2007 版的 MS Excel ，利用已知疾病的患病率和 95%置信水平，根据文献中提供的公式，通过不同的绝对精度来计算绘制诺模图所需的值。通过适当排列线条和距离来获得诺模图，使其符合该公式。这个诺模图可以方便地用于确定具有所需精度和 95%置信水平的诊断测试的灵敏度或特异性的样本量。只需将 0.70 和 1.75 分别乘以 95%置信水平下获得的数字，即可获得 90%和 99%置信水平下的样本量。只需移动标尺，诺模图就可以立即提供所需的受试者数量，并且可以在不重新计算的情况下重复使用。这也可以应用于反向计算。这个诺模图不适用于检验假设设置，仅适用于诊断测试和金标准结果都具有二分类别时。
