[_a_a_a_]

'perplexity'?

[esafak]

https://en.wikipedia.org/wiki/Perplexity

[IanCal]

The wiki link is good. In the context here it's easy to picture it as how weird a sentence would sound to a native speaker. Low perplexity means what was generated would be unsurprising if you saw it in the dataset.

[_a_a_a_]

I'll check the wiki link, but how is perplexity different from the measure of Surprise, in terms of Shannon's stuff?