Good question! I suppose that the experiment wants to capture the actual 'information density' of the language, and hence looks at the full language. Then, the

Good question! I suppose that the experiment wants to capture the actual 'information density' of the language, and hence looks at the full language. Then, they want to avoid any modification in speech rate due to the semantics of the spoken text. This does not make sense for a hypothesis where the actual bit-rate of speech tends towards 39 b/s. That is, when your text happens to convey more bits, you slow down. However, for an alternative hypothesis, this design does make sense. The idea here is that a language naturally converges to a speech-rate that gives 39 b/s. The idea here is that the actual speech-rate is much more constant, and just drops until it becomes too fast. For that, I'd argue you don't want the mean bit-rate but something like the 90th percentile bit-rate. Because it seems to me that speech-rate that is 'too fast' more than 10% of the time would not really be natural.