How High a Degree is High Enough for High Order Finite Elements?

High order finite element methods can solve partial differential equations more efficiently than low order methods. But how large of a polynomial degree is beneficial? This paper addresses that question through a case study of three problems representing problems with smooth solutions, problems with steep gradients, and problems with singularities. It also contrasts hadaptive, p-adaptive, and hp-adaptive refinement. The results indicate that for low accuracy requirements, like 1 % relative error, h-adaptive refinement with relatively low order elements is sufficient, and for high accuracy requirements, p-adaptive refinement is best for smooth problems and hp-adaptive refinement with elements up to about 10 degree is best for other problems.