A magnetron sputtering discharge is studied by optical emission and absorption spectroscopy, following an experimental design procedure. The 33-mm-diam target is made in an aluminium base alloy. The radiation is focused on an optical fiber and is spectrally analyzed in a Jobin-Yvon computer-controlled 1.0 m Czerny-Turner scanning monochromator. The spatial resolution is 2 mm. Emission line intensities from the Ar sputtering gas and the sputtered species are studied as a function of geometrical and physical discharge parameters : excitation mode (rf or dc), target thickness, gas discharge pressure, electric power (rf) or current (dc), and the distance from the target, In emission spectroscopy, two Ar I lines are studied : Ar I 425.9 nm 3p(5)5p(3p(1)) -3p(5)4s(1s(2)) and Ar I 451.1 nm 3p(5)5p(3p(5))-3p(5)4s(1s(2)) and also an Ar II line 442.6 nm 3p(4)4p(D-4(0))-3p(4)4s(P-4). The first excited level configuration of Ar (3p(5)4s) contains two metastable (1s(5),1s(3)) and two resonant (1s(4),1s(2)) levels. The Al excited and ground states have also been analyzed : the Al I 3p(2)p(0)-4s(2)s and Al I 3p(2)P(0)-4s(2)S. By use of hollow-cathode lamps, absorption due to aluminum atoms and argon metastable and resonant atom states densities are evaluated, as a function of the same parameters as in emission. The experimental responses (emission and absorption intensities) are fitted by a quadratic equation in which interactions between factors are included. The influences of the main factors are deduced and discussed.