More reusable cups, three different changes
1A fictional reading club records the cups used at two events. At the first event, 100 drinks are served and 10 are served in reusable cups. At the second event, 200 drinks are served and 30 are served in reusable cups. Each drink uses one cup, and the records cover all the drinks at each event. These are practice figures, not findings from a real club.
2The share of drinks served in reusable cups rises from 10 percent to 15 percent. The difference is five percentage points. Relative to the original share of 10 percent, the new share is 50 percent higher. Neither statement says that only five additional reusable cups were used. The number of reusable cups rises from 10 to 30, an increase of 20 cups.
3A draft report says, 'The share rose by five percent, so the number of reusable cups rose by five.' The editor asks the writer to name the quantity before doing the calculation. A difference between two shares is not the same as a relative change in a share or a change in a count. The club introduced a reminder before the second event, but these two records alone do not establish that the reminder caused the increase.