The Questions: How many grammar and punctuation errors are in practitioners' documents and students' papers? Is there a difference for students at the senior level and junior level? How do the errors affect meaning?
Results for a study of practitioner reports, senior-level student reports, and junior-level lab reports:
There is a BIG difference in the number of errors! The differences were statistically significant across the groups overall (p < .000, tested with a Kruskal-Wallis One-Way Analysis of Variance). Pairwise, there were significant differences between the practitioner writing and senior-level writing (p < .000) and between the practitioner writing and junior-level writing (p < .000), but not between the senior-level and junior-level writing (probably because of the large amount of variation within those groups).
The impact of the errors differs too. The majority of practitioner errors involved punctuation (about 50%), primarily commas. They rarely interfered with meaning. For example, there should be no comma after "two" but the meaning is still clear:
The building will house two, diesel-powered, 2MW engine-generators and will include electrical and storage rooms. [Practitioner report]
Students' errors, besides sometimes being annoying for their frequency, more often also interfered with meaning. For example, the following sentence from a traffic analysis has an extra clause ("as well as...") which is ungrammatical and conveys the opposite of the intended meaning:
Departures tended to have less pronounced localized peaks than arrivals, suggesting that departures are slightly less dependent on class time, as well as may account for the varying duration of class times (see Appendix Graphs A1 and A2). [Senior-level student report]