I should have added to the above that how the piece is performed and contextualised is also most relevant. A music hall song with a known composer sung in a music hall setting is not I think trad, one sung in a folk club or festival is. A Burns song sung in white linen and tartan sash using a trained voice is not trad, a Burns song sung in a 'natural' singing voice at a TMSA festival competition is trad. (How traditional singing copmpetitions are is of course another matter.)