I want to account for the fact that different levels of education are associated with different types of accreditation. For example, the impact of the change from 10 to 11 years of education may be different than the change from 11 to 12 years of education, due to the fact that individuals with 12 years of education tend to have received a high school diploma. I'm considering accounting for this impact in my regression. Why might I choose a step function rather than the standard (non-spline) polynomial regressions?

Please don't hand write. thanks.