Evaluate the effectiveness of a set of CS lab exercises for education
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.