I am reading an article about detection of differential methylation. They use the term "cell type composition" in the following context:
"Researchers need to carefully design studies that associate phenotypes with DNA methylation. Some aspects, such as cell type composition, cannot be readily controlled by design; patients and therefore individual DNA samples simply differ in their cell type composition." (Robinson, MD., 2014)
I would like to know to what "cell type composition" refers?
Does it refer to the composition of a specific cell-type in terms of its methylation pattern (or can it refer to something more general outside this context)? It makes sense to me that different cell-types should have different methylation patterns, that's why there are different cell-types in the first place. But is it that even within one specific cell-type, the composition of methylation pattern is different across different individuals?