方便取樣和立意取樣之比較

Comparison of Convenience Sampling and Purposive Sampling

方便取樣和立意取樣是不相同的取樣方式，本文將簡介目標母群體（target population）、可及母群體（accessible population）、簡單隨機取樣（simple random sampling）、所要的樣本（intended sample）、實際樣本（actual sample）、取樣（sampling）、統計檢力分析（statistical power analysis），並從這幾個面向來比較並釐清「方便取樣」和「立意取樣」之不同。方便取樣是非機率取樣，是研究者從可近母群體中找到容易、就近方便得到的研究對象，可用於質性研究或量性研究，一般用在量性研究。這些容易被研究者找到的研究對象，會有較多參與研究的機會，因此母群體中的成員參與研究的機會是不均等的，研究結果也就不能代表母群體。立意取樣一般用於質性研究，研究參與者是研究者從可近母群體精挑細選出來的，每個研究對象因其特質不同，提供給研究者不同深度的資料，非任何一研究可近母群體的成員就能夠取代，也並非是就近或容易找到。立意取樣的樣本數目不似量性研究用統計檢力分析（statistical power analysis）去計算，而是當資料飽和即可停止收案。

Convenience sampling and purposive sampling are two different sampling methods. This article first explains sampling terms such as target population, accessible population, simple random sampling, intended sample, actual sample, and statistical power analysis. These terms are then used to explain the difference between ”convenience sampling” and ”purposive sampling.” Convenience sampling is a non-probabilistic sampling technique applicable to qualitative or quantitative studies, although it is most frequently used in quantitative studies. In convenience samples, subjects more readily accessible to the researcher are more likely to be included. Thus, in quantitative studies, opportunity to participate is not equal for all qualified individuals in the target population and study results are not necessarily generalizable to this population. As in all quantitative studies, increasing the sample size increases the statistical power of the convenience sample. In contrast, purposive sampling is typically used in qualitative studies. Researchers who use this technique carefully select subjects based on study purpose with the expectation that each participant will provide unique and rich information of value to the study. As a result, members of the accessible population are not interchangeable and sample size is determined by data saturation not by statistical power analysis.