Research design is pretty much strict in its guidelines because the main intention is to establish parameters that will work to a) to ensure the validity of the data and b) to preserve the right to confidentiality of the participants. These are just some of the many things that research design, both qualitative and quantitative, aim to achieve during an investigation.
Students with disabilities are no different than the typical participant when it comes to protecting their confidentiality. However, other things must be considered due to their specific needs.
First, the intervention that is applied must go in tandem with the student's IEP. Also, there should be a lot of research done prior to any study concerning the potential consequences that may occur as a a result of the intervention. As with any regular participant, bias and assumptions should be eliminated as to avoid assumptions about the conclusion of the investigation.
However, students with disabilities do have a lot of laws protecting their rights. Special consideration must be given to clearing permissions from guardians, doctors, and other professionals who work with the student. That, however, belongs to logistics alone.
As far as the positive influences in research design is it safe to argue that students with disabilities have opened the door for researchers to explore in depth the need for valid and relevant research. The fact that their confidentiality and rights to a free and non threatening educational setting are imperative to the research design has made this same need equal to all participants.
In all, students with disabilities have done nothing but open doors to the field of research and it is clear that a lot of improvements have been made on their behalf. Hopefully, this pattern will continue its due course.