The second constraint implies that the coefficients for each independent variable across the three equations must sum to zero. Additionally, the equations must be slightly different, so instead of using identical independent variables, we include an extended market work model that includes labour market access as proxied by distance from home to office. We also assume that the error terms are independent and identically distributed (iid) across the activities with a mean of zero and homoscedastic variance £ = E (uj u'j |X).