The Unilever annual review stated: 

Total Shareholder Return (TSR) is a concept used to compare the performance of different com-panies’ stocks and shares over time. It combines share price appreciation and dividends paid to show the total return to the shareholder. The absolute size of the TSR will vary with stock markets, but the relative position is a reflection of the market perception of overall performance relative to a reference group. The Company calculates the TSR over a three-year rolling period . . Unilever has set itself a TSR target in the top third of a reference group of 21 .  companies.

Discuss (a) why a three-year rolling period has been chosen, and (b) the criteria you consider appropriate for selecting the reference group of companies.