The QWERTY arrangement was created in cooperation with educator Amos Densmore who studied the letter-pair frequency. It seemed if two type bars were near each other in the circle, they would tend to clash into each other when typed in succession. So, Densmore and Sholes figured the most common letter pairs such as "TH" and separated them at safe distances in the circle -- Wally Hauck: Stop the Leadership Malpractice, Motivational Press (2015).
Mr. Amos Densmore was not an educator at that time in 1860's. He was then a proprietor of Densmore Oil Company at Meadville, Pennsylvania, which manufactured train cars for transporting petroleum (cf. U. S. Patent No.53794). Furthermore, "E" and "R" are placed near each other in the typebasket circle as I mentioned before, while "ER" is one of the most common letter pairs in English.