good; 7 = very poor). Although there are a variety of problems with the use of Likert‐type ratings, there is a long history in speech‐language pathology of quantifying subjective phenomena using this approach (i.e., (Darley et al., 1969). One advantage of subjective measures is that a variety of dimensions of speech may be reflected in the numerical rating, and it is possible that intelligibility measures obtained from such a rating may reflect a more holistic view of intelligibility than transcription‐based scores.
Perhaps the most widely referenced screening guidelines and the most widely used clinical tools for speech intelligibility development in children are based on subjective ratings of intelligibility made by parents or other familiar communication partners. For example, Coplan and Gleason (1988) identified cut points for typical intelligibility development in children between 12 months and 5 years of age by asking parents to make a forced‐choice categorical rating regarding how much of their child’s speech they thought a stranger would be able to understand. However, these subjective parent ratings have not been validated with objective measures of intelligibility, thus the extent to which parents rate their child accurately relative to some objective standard is unknown.
More recently, McLeod and colleagues (McLeod, Crowe, & Shahaeian, 2015; McLeod, Harrison, & McCormack, 2012) developed the Intelligibility in Context Scale (ICS) to characterize intelligibility of children across different communication partners and contexts, as revealed by parent ratings. The ICS asks parents to rate their perception of their child’s intelligibility on a 5‐point scale across seven different contexts. Studies of the ICS have examined its relationship with segmental measures such as percentage consonants correct, percentage vowels correct, and percentage phonemes correct, as scored on standardized tests. The ICS has not been examined relative to other measures of intelligibility to our knowledge. It is widely used and has been translated into more than 60 languages (McLeod et al., 2015), however, normative data are limited and growth curves have not been developed.
In a recent study, Natzke and colleagues (Natzke, Sakash, Mahr, & Hustad, 2020) examined measures of intelligibility including percentage of intelligible utterances, parent ratings of intelligibility, and multiword transcription intelligibility scores from elicited utterance, all obtained from the same children at three longitudinal time‐points. Their results found weak associations between measures, suggesting that different measures of intelligibility are not reflective of one another even for the same children. Results also showed that not all measures were sensitive to growth over time. Large‐scale studies of intelligibility using consistent methods across the full range of development are currently underway.
4.4 Intelligibility from a Developmental Perspective
Adult speakers without communication disorders are generally assumed to be fully intelligible. However, for children, acquisition of adult‐like intelligible speech is a protracted developmental process, beginning early in the first year of life with vocal play, babbling, and word approximations, and continuing through childhood. Segmental development (acquisition of speech sounds) has been well documented in the literature. Expected age of acquisition for consonants and vowels in single words has been characterized using expert perceptual techniques (McLeod & Crowe, 2018; Smit, Hand, Freilinger, Bernthal, & Bird, 1990b). These data have been very useful to clinicians for assessing children’s speech sound development and identifying children with speech sound disorders. Developmental data indicate that English‐speaking children produce most speech sounds accurately by about 5–6 years, with adult‐like mastery expected at about 8 years (Sander, 1972; Smit, Hand, Freilinger, Bernthal, & Bird, 1990a). However, studies suggest that intelligibility is not readily predictable from phoneme data (Ertmer, 2010; Weismer, 2008; Whitehill, 2002). For example, studies have indicated that measures such as percentage of consonants correct (PCC) have a weak relationship with intelligibility (Ertmer, 2010). Generally, the number of articulation errors is negatively correlated with intelligibility; however, individuals can have significant articulation errors and still be highly intelligible (Whitehill, 2002).
Although it is clear that children acquire intelligible speech gradually, the precise course of development of intelligibility in typical children and the range of expected variability over the full course of development is not well understood. Problems that have plagued the historical literature include methodological differences among studies, such as whether intelligibility was measured objectively or subjectively, whether listeners were “experts” (e.g., speech‐language pathologists or phoneticians or naïve listeners), and the nature of speech material (elicited vs. spontaneous; single words vs. sentences vs. discourse or conversation). These differences among studies have led to conflicting reports on intelligibility development. Across studies, findings are discrepant and difficult to reconcile, for example, intelligibility of 3‐year‐old children has varied for different studies between about 53% and 96% (Chin, Tsai, & Gao, 2003; Flipsen, 2006; Morris, Wilcox, & Schooling, 1995; Weiss, 1982). From the existing literature it is impossible to know whether these values reflect the range of variability among typical children or whether they are a result of methodological differences between studies. Data for children at 4 years of age and older are similarly discrepant. However, one consistent and important finding is that intelligibility increases with age.
Growth curves for intelligibility development based on a large sample of typical children producing elicited utterances (single word and multiword) transcribed by unfamiliar listeners have recently been published for children up to 47 months of age (Hustad, Mahr, & Rathouz, 2020). Results indicate that there is a very wide range of variability among children at 2 years of age, with 5th and 95th percentile single word intelligibility scores of 18% and 74%, respectively. However, variability reduces somewhat with age, with 5th and 95th percentile single word intelligibility scores of 55% and 86% respectively at 4 years of age. This variability sheds some light on previous studies showing very discrepant results, suggesting that the range of typical intelligibility development is very wide, particularly for younger children. Results from Hustad and colleagues (Hustad, Mahr, & Rathouz, 2020) indicate that there is an intelligibility advantage for single word production prior to 41 months of age; after 41 months of age there is an intelligibility advantage for multiword production. Otherwise, the range of variability for typical children is similar for single word utterances and multiword utterances. Notably, typically developing children are not 100% intelligible as indicated by objective measures at 4 years of age. In addition, ongoing preliminary work comparing intelligibility development in children who speak different native languages is revealing important convergences. These results suggest that there may be some intelligibility development universals across languages. Such a finding would have critical implications for early identification of functional speech deficits in children.
A key issue is the ability to differentiate between children whose intelligibility falls within the range of age‐level expectations from those whose performance is delayed or disordered with regard to age‐level milestones. For many children, intelligibility reductions beyond age‐expectations may have a significant detrimental impact on functional communication and on social participation, leading to important negative educational consequences, since speech is a primary modality through which children in the early grades demonstrate their learning. Accurate differential diagnosis of intelligibility deficits, early identification, and treatment to improve intelligibility is critical for these children.
Studies are currently underway that seek to identify cut points for typical intelligibility development and to validate the diagnostic accuracy of intelligibility cut points for separating children who have mild or subtle speech motor disorders from those who are in the lower percentiles of typical development. Recent work employing receiver operating characteristic (ROC) curves (DeLong, DeLong, & Clarke‐Pearson, 1988) suggests that intelligibility scores differentiate between children with cerebral palsy (CP) who have speech motor impairment and typically developing peers with nearly perfect certainty (area under curve = .99). These data further suggest that at 5 years of age nearly all typically developing children had intelligibility scores above 87%, while the vast majority of children with CP and speech motor impairment had intelligibility below 72% at the same age (Hustad, Sakash, Broman, & Rathouz, 2019). These findings are consistent with earlier work suggesting that the range of intelligibility