[[Image:Gruppenbeitragsmethodenprinzip.png|thumb|Principle of a group-contribution method]]
The Joback method is a [[group-contribution method]]. These kindkinds of methods use basic structural information of a chemical molecule, like a list of simple functional groups, addsadd parameters to these functional groups, and calculatescalculate thermophysical and transport properties as a function of the sum of group parameters.
Joback assumes that there are no interactions between the groups, and therefore only uses additive contributions and no contributions for interactions between groups. Other group-contribution methods, especially methods like [[UNIFAC]], which estimate mixture properties like activity coefficients, use both simple additive group parameters and group-interaction parameters. The big advantage of using only simple group parameters is the small number of needed parameters. The number of needed group-interaction parameters gets very high for an increasing number of groups (1 for two groups, 3 for three groups, 6 for four groups, 45 for ten groups and twice as much if the interactions are not symmetric.).
Nine of the properties are single temperature-independent values, mostly estimated by a simple sum of group contribution plus an addend.
Two of the estimated properties are temperature-dependent: the ideal-gas [[heat capacity]] and the dynamic [[viscosity]] of liquids. The heat-capacity [[polynomial]] uses four4 parameters, and the viscosity equation only 2. In both cases the equation parameters are calculated by group contributions.
