Evaluate the effectiveness of a set of CS lab exercises for education I am a PhD student studying wireless networks/telecommunications, and I have developed a set of lab exercises related to the field. We have used them with > 100 students so far in B.S. and M.S. classes on computer networks, wireless networks, and wireless communications.

With the first few cohorts, I asked them to rate each exercise with respect to:


overall rating
difficulty of lab exercise
how interesting the material was
experiment design


and to rate self on knowledge of topic before and after the lab. I also ask for general open-ended feedback ("How can we improve this exercise?" "Any other comments?") and recently introduced automated systems to measure exactly what students are doing in the lab. This gave me a general sense of which exercises were interesting to students, and which they thought were useful. Now I am looking for a way to evaluate how effective the labs are at educating students - not just their popularity. 

I've been doing some reading in CS education research (although education research is not my field, so I can't devote too much time to getting up to speed on all the methodology), but I haven't come across a study design that I think would be a good fit for my scenario. I am under some constraints:


I'm not the instructor for the class. 
I just run the lab exercises via a website. I don't meet the students in person. 
I don't have the ability to set up a "control group" by offering one section of the course with the lab and one section without. 
I can't ask my students to do a lot of work that is only for purposes of methodology assessment (e.g. I can have them fill out a short pre-class survey, but not much more then that). I can ask them to do some things that are also learning assessments (like quizzes on the lab topic).



  Given these constraints, how can I effectively evaluate the educational tools I've developed? (specifically, learn whether they actually improve students' understanding of the course material)


Update: 

I don't have access to a comparable cohort from one year to the next, or between two sections. In the end, I came up with a set of questions that I think will help evaluate the exercises, even across only one group of students, all of whom are participating in the lab; I describe this in my own answer.
You are adressing a topic which (in my humble opinion) is one of the most difficult ones: How to measure effictivenes of teaching methods. 

The only good way is to have a control group (and make sure they are large enough, randomly asigned, subgroups represented equally, et.c). In most practical situations this is just not feasible. (I have the same problem at the moment).

What I plan to do (so it is just an idea, not proven to be correct - fedback is wellcome!):


ask students who did the class twice how they liked the new format and whether it helps them in learning
Compare outcome.


I think 2.) is the one we should aim for since the ultimate measure of successful teaching is compentent students leaving the university. Since it is hard to measure their competence late in real life situations, the only thing we can do is measure their perfomence by tests / exams. So I'll compare the results of my current group with the one from last year and look for failure rate, attendeance at test, ... Given that your tests are comparable, this gives at least an indication.