How does shielding effect change ionization energy?

And first some necessary definitions...the ionization energy is the energy associated with the reaction...

#M(g) + Delta_"ionization energy"rarrM^(+)(g) + e^(-)#

As always, this is a contest between nuclear charge, represented by #Z_"the atomic number"#, and shielding by other electrons. Now it is a fact that the nuclear charge is SHIELDED VERY INEFFECTIVELY by INCOMPLETE ELECTRONIC SHELLS. And this is illustrated by the observed ionization enthalpies...

...with the trend that ionization energies INCREASE across a Period, from left to right as we face the Table. but DECREASE down a Group, a column of the Periodic Table....witness the decrease in ionization energies observed for each alkali metal. Here, a new valence shell is commenced, and ionization of the valence electron is FACILE compared the elements which precede it on the last Period. As nuclear charge increases across the Period, the ionization energies become GREATER.....