Damping　reduction　factor　plays　a　central　role　both　in　scientific　literature　and　seismic　codes,　but　still　now　proposed　formulations　show　a　quite　large　scatter.　The　main　goal　of　the　present　work　is　to　explore　a　new　definition　of　the　damping　reduction　factor.　The　concept　of　stochastic　response　spectrum　is　adopted　in　order　to　predict　the　earthquake　response　of　a　linear　SDoF　system,　on　the　basis　of　the　random　vibration　theory　for　non　stationary　process.　The　peak　of　the　response　of　a　SDoF　system　under　a　non-stationary　stochastic　process　is　used　to　define　the　stochastic　displacement　spectrum.　The　damping　reduction　factor　is　thus　evaluated　as　the　ratio　between　the　maximum　displacement　of　systems　with　a　given　damping　and　a　conventional　one　subject　to　the　same　earthquake.