Semi-Additive: Balances and Snapshots

Your semi-additive answer: 'Semi-additive measures are the most dangerous because they SUM correctly across most dimensions. Balance summed across accounts on March 15 is correct. Balance summed across January through March is 90x the actual balance. The trap is that it LOOKS right until someone checks against the source.' Say 'looks right until someone checks.' That is what makes semi-additive measures dangerous. The Rule You Must State Immediately State the rule in one sentence: 'Balance is a stock, not a flow. Stocks cannot be SUMmed across time. Deposits are flows. Flows can.' If you say 'stock vs flow' unprompted, the interviewer knows you have studied this deeply. Then say: 'Across time, I use LAST_VALUE for point-in-time or AVG for average balance. Never SUM.' What You Can and Canno