We completely understand that your privacy is of utmost importance. Your test may contain sensitive or confidential data. Here at Userbrain our testers have already agreed to an NDA while signing up for our service:
 
7. Confidentiality of Tests
You agree that all Tests are confidential. You agree that you will not use, disclose or otherwise make known to third parties any Tests, recordings, or any associated information or the like, unless such information is publicly known or Userbrain has expressly permitted you to do so. This obligation shall apply during and, for an unlimited period of time, also after the end of the contract term. In the event of a violation of the obligation to maintain secrecy, the User undertakes to pay a contractual penalty in the agreed lump sum of USD 5000, which shall be irrespective of fault and independent of whether a specific damage has been caused or not. Such contractual penalty shall become due for payment at the time at which Userbrain obtains knowledge of the violation. Additional claims for damages or other claims of Userbrain or third parties shall remain unaffected thereby. The reasonableness of this contractual penalty is expressly recognized and shall be subject to no judicial right of reduction. You hereby agree to sign a separate non-disclosure agreement in the event that Userbrain or a client of Userbrain asks you to do so.
 
This being the case, there is no need to add on a signable/electronic NDA to your test.
You can find our full terms of use for testers here: Terms and Conditions