FRSales Volume Index

Hi ! Can an expert please tell me how the sales volume index has been calculated in the table below ? (sorry for spacing)

Paper Masking Tapes

£’000 £’000 £’000
Sales 2,500 2,400 2,300
Cost of sales 1,625 1,680 1,725
Gross profit 875 720 575
Transport costs 150 192 230
R & D moderate moderate moderate
Market share 20% 20% 20%
Sales volume index 100 106 112
Product range narrow medium medium

Thanks