It's just an idea but maybe the Feedback Console will be a good way to provide easily and simply feedback during or shortly after the presentation. Please have a look at attached picture. A common use case for this is to discover how people feel about a particular topic. Every time people leave the room they can easily press the button and leave the vote
Pros:
º no crowd - every time you leave the room you can easily press the button and vote
º no problems with the phone/the app
º everyone who participate in the session are eligible to vote
º easy to access for all of the people
º easy to access all the time (especially in the exit)
Cons:
º no security (if someone press the button several times but there can be one supervisor/volunteer at the end of session)
º no fruitfull feedback - only opinion mining & statistics (how many people voted, how many were happy or not etc.)
º costs (?)
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