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If you want to launch individual surveys using different settings, then you should not use the Launch button at the end of the New Unit Assessment wizard. Instead, you should publish the assessment, then go back in and individually launch each survey.
See also: Sheet Settings
Survey options for individual surveys
If you choose to publish an assessment instead of launching at the time of creation, you can come back later and launch the individual surveys (see also: HOWTO: Launch a survey ). The survey launch options look very similar to the launch options for the entire assessment.
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All of the settings are the same as explained above. The difference is that on this dialog window, you are only specifying launch settings for this one survey.