Figure 6.

Estimated eigenfunctions at level 1 (Rows 1-2) and level 2 (Rows 3-4). In Row 1 and 3, the solid lines represent estimated eigenfunctions. In Row 2 and 4, solid lines represent the overall mean function μ(t), and the lines indicated by “+” and “-” are mean function plus or minus a multiple of ${\varphi }_1^{\left(1\right)}\left(t\right)$, respectively. Namely, “+” represents $\mu \left(t\right)+c{\varphi }_1^{\left(1\right)}\left(t\right)$, and “-” represents $\mu \left(t\right)+c{\varphi }_1^{\left(1\right)}\left(t\right)$ for some constant c.