Chapter 10
Data Sufficiency
Determining whether a question can be answered from the data given
10.1 The principle
Before calculating, ask: do I have enough data for a unique answer? If any key value is missing or ambiguous, the answer is 'impossible to determine.'
10.2 The two-unknowns trap
A + B = C (known) A = ? B = ? → Cannot be solved — need one more equation
Example: total revenue €9.4M. North = €3.8M. East + South = €5.6M. Without East or South individually, neither can be determined.
10.3 Annual average vs monthly detail
💡 Knowing the annual average rose from 5.4% to 6.1% tells you the year-on-year trend, but says nothing about specific months.
10.4 Operating margin requires ALL operating costs
Operating margin = Operating profit ÷ Revenue Operating profit = Revenue − ALL operating costs
Staff costs alone are insufficient. Without rent, depreciation, materials and other overheads, operating margin cannot be calculated.
10.5 Weighted average requires all the weights
Without respondent counts per region, the true weighted average cannot be determined. A simple average of regional scores treats all regions as equally sized — which is almost always wrong.