Though it is clear that both mean and stdev are to be calculated I suggested a confusing name for your one one use case. I should have suggested you name it "calculate mean and standard deviation.
I had intended to set the problem so that it was clear that the visitors calculate mean and stdev per Field. Since it only makes sense to average stuff of the same dimension I imagine you have already figured this out. The ambiguity was unintended.
It looks like one sequence diagram is going to be a bit long, so after class I suggested to a few students that they could break it up into two pieces. The first page would illustrate how iteration works and the second could handle visitation.
That's right. Just paper to the ta of your section.