  It has recently been shown that a simple empirical formula, in terms of the
mass number and the valence nucleon numbers, is able to describe the main
trends of the lowest excitation energies of the natural parity even multipole
states up to $10^+$ in even-even nuclei throughout the entire periodic table.
In an effort to understand why such a simple formula is so capable, we
investigate the possibility of associating each term of the empirical formula
with the specific part of the measured excitation energy graph.
