1. An apparatus for determining a rate to be applied in respect of a data set comprising a value for at least one variable attribute in an electronic system or process, comprising: an input ( 105 ) for receiving the data set; a rate modelling component ( 115 ) arranged to store a model comprising: (i) data ( 205 , 210 ) defining a rating space having at least one dimension defined by an attribute of the electronic system or process; (ii) a rating vector definition comprising at least one rate parameter; and (iii) data ( 225 , 230 ) which defines distinct regions in the rating space ( 205 , 210 ) over which the defined rating vector is invariant, and which defines the respective invariant rating vector ( 235 ) for each distinct region; and rate determining means arranged, on receipt of the data set, to identify a corresponding one of the distinct regions ( 225 , 230 ), the respective invariant rating vector ( 235 ) and hence the rate to be applied in respect of the data set.