Quantitative Reasoning and Data Interpretation
What the exam tests
Quantitative Reasoning and Data Interpretation tests calculation under time pressure and judgment about what a number means. Expect percentages, ratios, weighted averages, rates, growth, probability, and tables described in words.
Core rules
- Percentage change = (new - old) / old; percentage points compare two percentages directly.
- Weighted average = sum of (weight x value) / sum of weights. Never average subgroup percentages unless weights are equal.
- Ratios need a common base. Gross margin = (revenue - COGS) / revenue; mark-up = gross profit / cost.
- CAGR = (ending / beginning)^(1/n) - 1.
- Correlation does not establish causation; check sample size, selection, and missing data.
- When a question asks for an approximate answer, keep one guard digit until the final step.
Worked example
A survey has 200 small firms and 80 large firms. 60% of small firms and 45% of large firms report a training budget. Small-firm count = 120. Large-firm count = 36. Combined rate = 156 / 280 = 55.7%, not the simple average of 60% and 45% (52.5%). The larger small-firm group pulls the combined rate upward.
Traps
Watch numerator and denominator definitions. A 20% increase followed by a 20% decrease returns to 96%, not 100%. Do not add percentages with different bases. In data interpretation, read units and time periods before calculating. Base-rate neglect is common in probability questions.