Another interpretation of the results is that, it being an introductory course, was attended by people who had prior knowledge and some who had not been exposed to the content before. Those who rated you well may have been from the former camp, and found the material easy. Those who were from the later may have found your teaching to be less than effective.
On the other hand, the opposite could have been true. Take, for example, a business school that requires their Information Systems majors to take an introductory "Information Systems" class that is also required of all business students. The material could, at best, be remedial or even detrimental to covering more advanced topics in the same field because of the introductory course. (For example: Group projects where they end up doing most of the work).
Both of these scenarios are plausible. Another user mentioned getting evaluations throughout the semester. That's good, but you should also collect information about who the students are to make better use of those evaluations.
Do the students have prior experience in your subject?
- For those with prior experience, how are they receiving the material? Is it beneficial to them? Are you essentially having a few students do a large portion of the classes work? (E.G. From the 2nd example, is there an IS major in every group (if there are group projects) who is doing 80-100% of the group work?)
- For those without prior experience, is your material helping them? Are assessments enhancing their learning as well? Or are they spending disproportionate amounts of time on the work relative to those with prior experience?
If you can answer these questions, you can better figure out which group of students is giving you evaluations that are positive and which groups are giving you the negative evaluations. From there, you might determine that the evaluations you are giving are too easy/too hard relative to a large portion of the class. Maybe you don't even need to make a decision then. If your findings are that you have a hugely disparate class, maybe your department changes the structure of the course such that more advanced students take another class and more remedial students take another.