How to grade students' lab worksheets to prepare them for a final lab exam? I am a physics lab TA and I was recently informed that my engineering students will receive a lab final at the end of the semester worth 50 percent of their grade. The final exam is not personally created by me, but is decided amongst all of the lab TA's teaching the same course in the same semester. The meeting amongst the TA's will occur next week, however I am attempting to prepare for whatever will come, for better or worse for my students. I want my engineering students to succeed, and I want to give them all of my resources to give them the preparation to pass the final exam. I assume that I will not be allowed to tell my students what the lab final will be, or what types of questions will be asked. My students are required to complete 9 weekly lab worksheets corresponding to their specific lab of the week. All sum total of all lab worksheets are worth the other 50 percent of the lab grade.

One issue I am currently weighing in my head is how to grade the lab worksheets. Should I go easy on the grading to soften the blow of the final exam, or should I grade harshly to force them to correct every mistake they make in prepration for the final exam?
While I don't TA physics, but Geology, our labs tend to have the same structure. Luckily the labs are small enough that I can get to know the groups ect, So I can determine how hard they are working on the labs ect, so I generally grade 70% effort and 30% correctness. The difference is though, that you are teaching engineering majors, where I tend to teach non geology or STEM majors. Getting something in the lab 100% correct for my students isn't as important and following the scientific process, but, for a class of engineers, I would have higher expectations since the content they are learning is helpful for their careers later. IE, they should have a vested interest beyond the grade. 

My experience with the Lab exams is that the students either do very well or very poorly. I would be somewhat lax on the labs, especially on first time mistakes. But there isn't much you can do if they aren't working hard and aren't completing the labs correctly and they will likely fail the test if thats the case anyway.