David Gerard wrote:
Does anyone have or know of *actual data* (rather than hypothesis or anecdote) on whether non-visual captchas are any good as yet?
To address one particular type: a conference paper reports experiments that suggest using speech synthesis degraded by noise is not an effective basis for audio CAPTCHA:
"...although there seems to be a gap in the ability of understanding synthesized speech with background noise between humans and computers, our results discourage using this gap to build an audio-based CAPTCHA"
Only the abstract is available online for free. The full paper would require an online purchase or a library visit.
Tsz-Yan Chan. "Using a Text-to-Speech Synthesizer to Generate a Reverse Turing Test," /ictai/, p. 226, 15th IEEE International Conference on Tools with Artificial Intelligence (ICTAI'03), 2003. **http://doi.ieeecomputersociety.org/10.1109/TAI.2003.1250195
Though the paper is from 2003, one would guess that particular CAPTCHA schemes get less, not more, effective over time as automated recognition technologies improve.