My theory is that an improvement required is a problem where you get main portion of the weigheted problem correct, as you
Pure speculation on my part, too and I will refer to the "Lateral" portion of the exam. Since most of the essays require a loads calculation (wind or earthquake), if you have errors at this part, everything else is based on the loads calculation and it will render your whole essay "unacceptable". On the other hand if you do OK with the loads calculation and design but miss on the detailing part, you might get a "needs improvement" score.
I agree with this assessment on the needs improvement part. I know on one problem in the afternoon I had the main portion correct, but had a brain fart on the rest. I did not complete the problem but received an improvement required on that problem. I'm still frustrated with the vertical concrete essay problem which I thought was cake and had worked out examples in multiple references but received an unacceptable on it. I'm going to go back to my original thought on passing the vertical or lateral portion is that you have to pass the morning and the afternoon components separately to pass. It does not make sense to get 33/40 on MC and 3/4 Acceptable on the essay and not pass with a minimum combined score of 33 + 23 = 57/80 Assuming minimum 7 points for an acceptable answer and some credit (2 Points) for an unacceptable answer. Looks like I will have to take a bridge class. ( I hate AASHTO)...