Is it ethical to have manditory questions in an online CrowdSourced survey with paid particpants? Our School Research Ethics Committee requires that when designing an online survey that each question is optional for the participants. These rules were devised before the era of CrowdSourceing when researchers had to rely on the goodwill of others to gather the necessary responses. 

However as I am paying each participant to complete this survey (using Amazon Mechanical Turk) do you think I have a case to argue that I should be able to make some, or even all, the questions mandatory? 

I have not approached the Research Ethics Committee about this yet, I just wanted to see what the norm is in other countries and institutions.

Thanks
Mandatory is a difficult concept. I think it is ethically questionable to not allow a participant to withdraw from a study at any time. There may be edge cases, for example drug studies, where further follow up treatment is required, but even here I think that treatment would not be considered part of the study. When a participant withdraws from a study we still need to provide them with whatever inconvenience allowance was agreed. We are not even allowed to prorate the inconvenience allowance, however, we break our longer studies into hour long segments and pay an hourly inconvenience allowance. It is perfectly reasonable to use a "forced choice" response system where participants are required to give a response in order to continue with the study. Using the Mechanical Turk, I think ethically you should provide an easy means of dropping out that is constantly available that will not penalize the participants. When it is so easy to withdraw, forcing them to answer a question which is not inherently necessary, seems silly. If they do not want to answer it and they have no other option, they will just withdraw.

Consider two experiments that consist of two questions. In the first experiment you either show either a circle or a square that is either red or blue. If for whatever reason the participant does not want to tell you what shape they saw, you can still get meaningful information about the color and therefore it would make no sense to force them to answer the first question. In the second experiment you show them a face and ask if it is a man or a women and based on that response you ask them if it was either Alice or Carol or Bob or Dave. For this experiment they need to answer the first question.