On intelligibility: why use it, and options for measurement

It’s been suggested that we use intelligibility as part of comprehensive speech assessment, and measurement of treatment outcomes. Why? Well, because intelligibility is kind of the point of speech therapy in the first place, right? Also, intelligibility can pick up on phonological changes that other measures (like percent consonants correct, PCC) can’t.

So which intelligibility measures are we supposed to be using, exactly? Or, more appropriately—what are our options?

First, there are many ways to measure intelligibility. We can use rating scales, single word measures, or connected speech; and raters may include the clinician, family, peers, or unfamiliar listeners. Each of these have their own pros and cons in terms of reliability, validity, and compatibility with clinical practice. But the gold standard has been to calculate the percent of words understood, by unfamiliar listeners, in a connected speech sample (Gordan-Brannan & Hodson, 2010).

And while speech samples + few unfamiliar listeners as raters may be ideal, that carries a time burden for clinicians. Further, we also really want data on how the child is functioning in his or her everyday life. These considerations are what make the Intelligibility in Context Scale (ICS) particularly enticing—developed and measured over the past several years from (ongoing) research by McLeod and colleagues.

What is the ICS? It’s a brief, 7-item rating scale, completed by the parents of preschool and school-aged children. It can supplement other clinical measures for a nice look at functional speech. The scale can be found here (also on the last page of this article). Additional things to know about it:

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Multilingual populations: It’s been translated into 60 languages (free, online!), and being multilingual doesn’t affect the scores (McLeod et al., 2015). It’s recommended that you use a separate sheet for each language the child speaks.

Screening: Use for preschool screening would be appropriate, especially as additional normative data is collected by future research. For now, this article can help you identify appropriate scores for your environment. You’ll extrapolate to your clinical population by looking at the scores they found in their sample of 4- and 5-year-olds (see Table 3 of the study). Do keep in mind the limitations of their study (read Limitations section). But basically, the scores in this study are relatively conservative, so children are generally likely to require further speech evaluation if their scores are lower than that 2015 study.

Psychometric properties: You can find this data throughout several of their articles; in particular, this one, which provides support for the ICS as a valid and reliable measurement of preschool children’s intelligibility.

Though we started by looking at the McLeod et al., 2015 paper, research for this Throwback review ended up sending us toward several papers on intelligibility, linked out above. Enjoy!