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To add Custom logic to your Survey you can add a Custom Scripting/Scoring type question. Click on the Add New Question link under the Edit Survey page:
In the Question Wizard pop-up from the Advanced Questions Types option select the Custom Scoring/Logic Script question type and click on the Next button.
Here you can enter the script for the required logic and save the question.
Logic Includes Branching: Enable this option if there is Branching involved in the script/logic
Logic Includes Looping: Enable this option if there is Looping involved in the script/logic (Please note that multiple rows of data are saved for each loop).
Enable Debug Information (For Testing): Enable this option for testing the script/logic
Enable Auto Submit: Sometimes you may use this question as the only question on the page. In such cases user will see a blank screen with continue button on the page. The same issue may occur if there is branching logic in the script/logic. If enabled the page is automatically submitted and user will not see the blank screen.