PHP Surveyor includes the capacity to generate individualised 'tokens', so if you have a list of people you want to invite to participate in a survey you can issue each one with a token, and they will be able to access the survey using that token. This allows for quite good quality control of your surveys.
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While this is a beta, it should be relatively stable.

Please download it and give it a try. Let me know if you find any bugs.

I'll be setting this up as a module project when I get a few minutes.

I've tried my best to limit the Xoops code to an absolute minimum so updating it with future releases should be relatively simple. Due to this though, you will need to use PHP Surveyors integrated template system for altering the actual forms. This shouldn't be a big deal as the templates are very simply HTML pages. You can use the integrated template system to make your changes. It will still appear within your Xoops theme.

You can download the Beta version at:

http://gaining-mass.com/projects/Surveyx%200.5%20Beta.zip

Have fun!