PM lisamarie23,
the part of my post which you say supports only a weak argument is actually sourced from Jon Barron and a leading health writer.
MEDSCAPE.
"To maximize the thoroughness and safety of the procedure, the colon must be completely empty prior to colonoscopy. Several options are available for precolonoscopy bowel cleansing".................... http://emedicine.medscape.com/article/1819350-overview#aw2aab6b3
So unless the medical purgative is effective in its cleansing, then the procedure would be pointless and the evidence for compacted fecal matter is removed beforehand.