How to ask for feedback from research students on my supervision abilities? As the lone experimentalist in my group, I end up supervising most of the students (M.S., B.S. and high school) who do experimentation-focused research in my lab.

I am always trying to improve my mentoring and supervision skills. So I would like to ask my current students for some feedback at the end of the summer.

However, I want to make sure they don't feel pressured in any way by this request, that they are assured there will be no negative or positive consequences to them, and that they understand that I really want honest answers. (I'm not fishing for compliments.)

And, I am looking for specific feedback that I can use to improve or build on, not just general complaints or reassurances that everything was fine.

Given the goals stated above, what's the best way to ask for this kind of feedback? Should we speak face to face in an "exit interview" kind of thing, or should I ask them to write something in an anonymous form online, or something else entirely?

What specific questions can I ask to get focused, helpful feedback on my supervision and mentoring abilities? 

Does anybody here have experience (as either supervisor or supervisee) with this kind of assessment, and have useful techniques to share?
Set the Stage

At the start of each relationship, let each student know that you value feedback, that you are adaptable/flexible in your approach to supervision, and that you will be asking for their feedback both during and at the end of the end of the supervision period.

Reinforce the Message and Values

During the supervision period, look for opportunities to demonstrate your adaptability, your interest in feedback, and it is OK for them to offer feedback even if they might feel a bit uncomfortable.  (Some people resist giving feedback to supervisors, including for gender/age/cultural reasons. They need to experience the process working successfully to overcome that resistance.)

Asking for Feedback

Aim for a face-to-face meeting, though it's fine if some people prefer to give feedback in writing.  Before you ask for feedback, first ask what is important to them in supervision relationship. You need to understand their frame of reference and values. Then ask: "What worked well for you? And what didn't?"  Then you can ask the feedback question: "For those things that didn't work so well for you, what would have worked better for you?"  You are asking how things could be different for them, within their frame of reference.  You aren't asking them to step into your shoes and advise you on how to be a better supervisor.