The phrase "substantial deviation" means a noticeable, significant, or important difference or change from what is expected or considered normal. Full definition
While I followed the basic ideas of his technique, I've made substantial deviations as well. (treesfullofmoney.com)
As the study progressed, we allowed the autonomously expressed interest of the group to adjust our course, which resulted in substantial deviation from the original set sequence. (reachinst.org)
[4] Subsequent observations revealed substantial deviations from the tables, leading Bouvard to hypothesize some perturbing body. (newworldencyclopedia.org)