Previous research has shown that continuous rating scales can be used to assess phonetic detail in children's productions, and could potentially be used to detect covert contrasts. Two experiments examined whether continuous rating scales have the additional benefit of being less susceptible to task-related biasing than categorical phonetic transcriptions. In both experiments, judgements of children's productions of /s/ and /θ/ were interleaved with two types of rating tasks designed to induce bias: continuous judgements of a parameter whose variation is itself relatively more continuous (gender typicality of their speech) in one biasing condition, and categorical judgements of a parameter that is relatively less continuous (the vowel they produced) in the other biasing condition. One experiment elicited continuous judgements of /s/ and /θ/ productions, while the other elicited categorical judgements. The results of Experiment 1 showed that the influence of acoustic characteristics on continuous judgements of /s/ and /θ/ was stable across biasing conditions. In contrast, the results of Experiment 2 showed that the influence of acoustic characteristics on categorical judgements of /s/ and /θ/differed systematically across biasing conditions. These results suggest that continuous judgements are psychometrically superior to categorical judgements, as they are more resistant to task-related bias.
Keywords: Acoustics; covert contrasts; rating scales; speech analysis; speech perception.