As of today I've had more than 2000 visitors to the blog who looked at the results of our research, and this has been great!
In Wiseman and Angliss' work (detailed in Part I of this series), they didn't find much of a result in the emotional scale scores on the survey, but they did find an increase in the written out "strange experiences" amongst the infrasound group. When I published the results from the survey here, we had read through the "unusual experiences" on our surveys and saw such a small amount of responses that we didn't think it likely that we would find anything statistically significant. And now, Dr. Riggins' analysis has confirmed that (details for statistics geeks here). Here's what she had to say:
I went through the questionnaires and rated the unusual experiences. The vast majority of respondents (over 150 out of our sample) didn't have any answer at all on this question or when they did answer said they didn't experience anything unusual. I did a t-test (test of means of two groups) of those who had reported any unusual experiences when infrasound was on versus when it was off. There was such a small sample size (13 responses among those who had infrasound off and 26 among those who had infrasound on) that it is not a big surprise that I didn't find anything statistically significant. However, it was a challenge interpreting the questionnaire comments.
I also performed a Chi Square analysis of a new variable that collapsed the responses into Yes (1) if they reported an unusual experience or No (0) if they didn't. As I suspected, nothing significant. If the parametric t-test that I did earlier didn't find anything, collapsing the variable into a nominal variable wasn't likely to find anything either. The nonparametric stats are more robust to problems with data, but less sensitive in finding differences. You just have less information when you have a nominal variable (a group like yes/no) than if you have a continuous variable (like 1, 2, 3).
Our results have been appearing on some paranormal websites claiming that we have discredited the idea that 19Hz sounds could cause people to have unusual experiences. Nothing could be further from the truth. Our experiment didn't find any results, but our experiment was also fundamentally different from all the other experiments done (of which I'm aware, anyway). In fact, there's a National Geographic program depicting a very similar experiment and they did claim some effect; you can view a video here (we unfortunately weren't aware of this experiment before doing our work, but I have since been in communication with the engineers who worked on that project).
There are many possible things that could explain why we didn't find anything statistically significant. Maybe our infrasound exposure was too short? Maybe we needed a stronger, scarier presentation? Maybe we needed a more emotionally suggestive environment (like a music concert)? All of these ideas could be taken further, and our findings can certainly assist future research.
Regardless of the results, I learned a huge amount during the process, and I have many more things I want to test. Thanks to everyone who helped, and everyone who has read this series!