In step 4, all the individual employment results are summed rather than combined using a harmonic series. This yields a count of all the employment jobs within the travel time threshold, adjusting for travel time using the negative exponential equation. This means the result can be interpreted as the number of co-located job opportunity equivalents where a value of one would be the equivalent of one home-based job. Again in the later steps (steps 5 and 7) a simple relative weighting is used to calculate the age group and overall values so the interpretation of the results is constant albeit the indicator value is significantly different from the indicator value of the consumed activities. There is no step 6 combining results from different activity types as the places of employment values are processed separately from the other activity results.
A limitation of the methodology is the outputs from the consumed and supplied activities cannot be combined because they use a different basis for calculating their respective metrics. Further work is required to make the supplied activities (employment) compatible with the consumed activities metric. In particular it would be worthwhile investigating the LUPTAI (Land Use and Public Transport Accessibility Index) approach (Pitot et al 2006) which applies a factor to the number of jobs, acknowledging only a small proportion of census employment jobs are actually ‘available’ to a household.