 The number of DOF equals the size of the stiffness matrix.The reader can see in the graphs that the convergence of the hp-FEM is much faster than the convergence of both other methods.Actually, the performance gap is so huge that the linear FEM might not converge at all in reasonable time and the quadratic FEM would need hundreds of thousands or perhaps millions of DOF to reach the accuracy that the hp-FEM attained with approximately 17,000 DOF.