Maybe I need a "you won't believe #7" in the subject line

As I mentioned the other day I'm conducting a post-road bike race survey. I've conducted a number of these and the response rate is usually in the 35% - 50% range (the stated preference data is are good enough for journals and I'm presenting another one of these papers at the SEA meetings). I get an email list of participants and send out a request to complete the survey via SurveyMonkey. For the first time I've sent the email out as HTML and that allows me to track who opened the email and who ignored it. After almost a week I'm surprised to find that almost one-third of the participants in the Blue Ridge Brutal have not opened an email with "Blue Ridge Brutal participant survey" in the subject line. I probably am an idiot for being surprised.