2.9. Grey relational analysis
Grey relational analysis (GRA), deduced by the grey system theory (Deng, 1989), has been broadly applied for evaluating the intrinsic interrelationships between multiple variables. GRA was used in this study and the grey relational grade (GRG) was obtained to assess the relationship between relative peak areas in the metabolite fingerprinting analysis and the antioxidant activity.
The GRA procedure involves the normalisation of original data and a GRG calculation. The 30 sets of data, representing the different PSP cultivars, including relative peak areas and antioxidant activities, were divided into three groups using the following methods: (1) grouping in order; (2) randomisation grouping by computer; and (3) arithmetic sequence grouping with the tolerance set to three. In the analysis, the antioxidant activity was set as the dependent variable, and the relative peak areas of thirteen common peaks were set as the independent variables. i and k mentioned in the following description refer to the ordinals of the common peaks (i equals antioxidant activity when i = 0) and the ordinals of the samples, respectively.
Normalisation of the original data was performed as follows. The average value was obtained from the following equation to make the raw data dimensionless: