The cumulative percentage of a total held by the items so far in a list sorted from largest to smallest; the column behind every Pareto chart.
Running share is the cumulative percentage of a total reached at each row of a list sorted largest first. Read down the column and it says how few items carry most of the total: the row where it crosses 80% is the Pareto cut-off. It is the same arithmetic as top-n share, shown for every n at once.
Sort the items descending. For each row, running share = sum of the values from the first row to this row ÷ the grand total. In Excel: =SUM($B$2:B2)/SUM($B$2:$B$11). The last row must read 100%.
Four customers with revenue of $400, $300, $200 and $100 (thousand) make a total of $1,000,000. Their running shares are 40%, 70%, 90% and 100%, so two customers hold 70%.
Computed on an unsorted list, which makes the curve meaningless. Run on child accounts instead of parents, which splits one large customer into several mid-sized ones. The full guide is Pareto analysis.