The majority of the time, we list radicals in order of stability based on two key factors. They are stability by resonance and stability achieved by adjacent atoms with lone pairs. Even so, the increase in stability is dependent on the atom’s electronegativity. As the electronegativity of an atom decreases, radicals can become more stable.
Alkyl radical stability increases as we proceed from
Methyl free radical < Ethyl free radical < Isopropyl free radical
More hyperconjugating structures are formed as we proceed from methyl to Isopropyl.