Grading Children with IEP

It would be very hard to grade someone with an IEP fairly in comparison with an average grade student for me. I feel I would be very bias when overlooking their paper and looking at their improvements rather than how accurately the work had been done. I am not sure if this is right or not. Should we compare their intellect equally with those of other students? Or should we compare their intellect with only themselves alone. If an average grade student did the same work as someone with an IEP how would I justify my lower grade for them to them? I still think I would be more lenient in my grading of a student with an IEP, but I am not sure if this is right or not. I wouldn't give them perfect scores for work that didn't have right answers, but I wouldn't fail them if I could see improvements and effort being done.