Date Published: June 20, 2018
Publisher: Public Library of Science
Author(s): Susan C. Weller, Ben Vickers, H. Russell Bernard, Alyssa M. Blackburn, Stephen Borgatti, Clarence C. Gravlee, Jeffrey C. Johnson, Andrew Soundy.
Sample size determination for open-ended questions or qualitative interviews relies primarily on custom and finding the point where little new information is obtained (thematic saturation). Here, we propose and test a refined definition of saturation as obtaining the most salient items in a set of qualitative interviews (where items can be material things or concepts, depending on the topic of study) rather than attempting to obtain all the items. Salient items have higher prevalence and are more culturally important. To do this, we explore saturation, salience, sample size, and domain size in 28 sets of interviews in which respondents were asked to list all the things they could think of in one of 18 topical domains. The domains—like kinds of fruits (highly bounded) and things that mothers do (unbounded)—varied greatly in size. The datasets comprise 20–99 interviews each (1,147 total interviews). When saturation was defined as the point where less than one new item per person would be expected, the median sample size for reaching saturation was 75 (range = 15–194). Thematic saturation was, as expected, related to domain size. It was also related to the amount of information contributed by each respondent but, unexpectedly, was reached more quickly when respondents contributed less information. In contrast, a greater amount of information per person increased the retrieval of salient items. Even small samples (n = 10) produced 95% of the most salient ideas with exhaustive listing, but only 53% of those items were captured with limited responses per person (three). For most domains, item salience appeared to be a more useful concept for thinking about sample size adequacy than finding the point of thematic saturation. Thus, we advance the concept of saturation in salience and emphasize probing to increase the amount of information collected per respondent to increase sample efficiency.
Open-ended questions are used alone or in combination with other interviewing techniques to explore topics in depth, to understand processes, and to identify potential causes of observed correlations. Open-ended questions may produce lists, short answers, or lengthy narratives, but in all cases, an enduring question is: How many interviews are needed to be sure that the range of salient items (in the case of lists) and themes (in the case of narratives) are covered. Guidelines for collecting lists, short answers, and narratives often recommend continuing interviews until saturation is reached. The concept of theoretical saturation—the point where the main ideas and variations relevant to the formulation of a theory have been identified—was first articulated by Glaser and Strauss [1,2] in the context of how to develop grounded theory. Most of the literature on analyzing qualitative data, however, deals with observable thematic saturation—the point during a series of interviews where few or no new ideas, themes, or codes appear [3–6].
Descriptive information for the examples appears in Table 2. The first four columns list the name of the example, the sample size in the original study, the mean list length (with the range of the list length across respondents), and the total number of unique items obtained. For the Holiday1 example, interviews requested names of holidays (“Write down all the holidays you can think of”), there were 24 respondents, the average number of holidays listed per person (list length) was 13 (ranging from five to 29), and 62 unique holidays were obtained.
In general, probing and prompting during an interview seems to matter more than the number of interviews. Thematic saturation may be an illusion and may result from a failure to use in-depth probing during the interview. A small sample (n = 10) can collect some of the most salient ideas, but a small sample with extensive probing can collect most of the salient ideas. A larger sample (n = 20) is more sensitive and can collect more prevalent and more salient ideas, as well as less prevalent ideas, especially with probing. Some domains, however, may not have items with high prevalence. Several of the domains examined had only a half dozen or fewer items with prevalence of 20% or more. The direct link between salience and population prevalence offers a rationale for sample size and facilitates study planning. If the goal is to get a few widely held ideas, a small sample size will suffice. If the goal is to explore a larger range of ideas, a larger sample size or extensive probing is needed. Sample sizes of one to two dozen interviews should be sufficient with exhaustive probing (listing interviews), especially in a coherent domain. Empirically observed stabilization of item salience may indicate an adequate sample size.