Table 2 Equations and abbreviations of pre-processing methods.

Difference model

${\text{x}}_i={\text{R}}_i^{\max }-{\text{R}}_i^{\text{min}}=\Delta {\text{R}}_i$

Fractional difference model

${\text{x}}_i=\Delta {\text{R}}_i{\text{ / R}}_i^{\min }$

Normalized fractional difference model

${\text{x}}_i=(\Delta {\text{R}}_i{\text{/R}}_i^{\text{min}})/{(\Delta {\text{R}}_i{\text{/R}}_i^{\text{min}})}^{\max }$

Difference model

${\text{x}}_i={\text{R}}_i^{\max }-{\text{R}}_i^{\text{first}}=\Delta {\text{R}}_i$

Fractional difference model

${\text{x}}_i=\Delta {\text{R}}_i{\text{ / R}}_i^{\text{first}}$

Normalized fractional difference model

${\text{x}}_i=(\Delta {\text{R}}_i{\text{/R}}_i^{\text{first}})/{(\Delta {\text{R}}_i{\text{/R}}_i^{\text{first}})}^{\max }$

Difference model

${\text{x}}_i={\text{R}}_i^{\max }-{\text{R}}_i^{\text{bp}}=\Delta {\text{R}}_i$

Fractional difference model

${\text{x}}_i=\Delta {\text{R}}_i{\text{ / R}}_i^{\text{bp}}$

Normalized fractional difference model

${\text{x}}_i=(\Delta {\text{R}}_i{\text{/R}}_i^{\text{bp}})/{(\Delta {\text{R}}_i{\text{/R}}_i^{\text{bp}})}^{\max }$