The A/B testing and user experience sessions are fine and good practices in general. I just really disagree that privacy settings or some "mixin value" that the average user has no idea what it does, makes any sense to expose at the UI level for 95% of users. You are likely to confuse or sow doubt much more than educate or help them.
I would rather see the dev team spend their limited time and resources on other much more important things than to try and test this. Of course so much depends on who your user base is when you bring them in for testing, but if you picked "real" users, such as soccer Moms and facebook teens, and grandmas, I feel very confident that none of them would a) want to think about what level of mixin to pick and b) would feel comfortable having to make that decision, and c) are likely to be scared of making a wrong choice, thus destroying the easy to use purpose of the GUI. Since there is no "right" answer, the software should hide the complexity from the user and pick a "good" value. Let the experts or advanced users worry about what mixin level matters to them. Grandma don't care (or want to care)!
Well, there are two different things you are trying to solve here.
1) Ease of use
2) Not having all transactions at the same mixin level
I don't think pushing the burden of 2 on to the average user is the way to solve it. Its a highly technical detail that 95% of users (or more) are not going to understand or want to think about, and it will constantly make them wonder if they are "doing the right thing" or if their transaction is secure enough. Could even discourage use.
The average user is completely ignorant and scared of technology. They know how to turn on their PCs, click the email icons, and that's about it. Thats not an insult to them. They are experts at their day job being doctors, lawyers, etc... but they just don't have any desire or understanding of tech in general. Given that the GUI wallet's purpose is to remove the complexity and make Monero accessible to the masses (if that's possible), then I would really focus on removing any "cognitive load" on the user. Don't force them to think about things that they can't possible understand anyway.
Perhaps a good solution would be to automatically randomize the mix in value between an upper and lower bound and not expose it at all to the end user. Advanced users that want to specifiy a certain value can do so in their preferences or an advanced options screen.
Once we have the entire interface completed we'll do interface testing with testing groups made up of people in multiple countries from all sorts of backgrounds, so we'll have a good cross-section. Typically they have a series of tasks to perform with no education or training on how to use the product or even what it is. We film them over their shoulder to see how they react and where they get frustrated. During this process we can do A/B testing with the privacy bar on/off between two groups, and then we'll have some tangible feedback as to what works and what doesn't. Should give us some direction as to what to do with that:)
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