Some measure of the market wage is generally taken as the benchmark. Three possible criteria come up here: (i) the global substitution method, in which the average wage for every kind of domestic activity is taken as a benchmark, (ii) the specialized substitution method, where the benchmark taken is the wage for the specific domestic activity, (iii) opportunity cost, whose benchmark is the wage that might be received by the person carrying out the domestic activity if they participated in the labour market. Is it valid to equate these activities, when they are carried out under very different conditions and norms?