Identify the incorrect statement among the following:

There is a steady decrease in the radii as the atomic number of the lanthanide elements increases. For every additional proton added in nucleus the corresponding electron goes to $4 f$ subshell. The shape of $f$-orbitals is very much diffused and they have poor shielding effect. The effective nuclear charge increases which causes the contraction in the size of electron charge cloud. This contraction in size is quite regular and known as Lanthanoid contraction. Since the change in the ionic radii in the lanthanide series is very small, thus their chemical properties are similar.
Related Theory There is no regular trend in ionization potential in the group due to lanthanide contraction. Caution The lanthanide contraction is responsible because $\mathrm{Zr}$ and Hf have about the same radius