Strategy: Data Interpretation and Counting Tactics

Reading Data and Choosing the Right Count

Strategy 1: Orient Before You Compute

On a Data Interpretation set, spend the first 15 seconds reading the title, both axis labels, the units, and any footnote — before looking at the questions. Most Data Interpretation errors are unit errors: mistaking a "% of total" axis for raw counts, or missing a "(in thousands)" note.

Strategy 2: Distinguish "Percent Of" from "Percent Change"

These are the two flavors of percent question, and they are graded differently:

• Percent of: $\dfrac{\text{part}}{\text{whole}} \times 100\%$.
• Percent change: $\dfrac{\text{new} - \text{old}}{\text{old}} \times 100\%$ — always divide by the original value.

An increase from 180 to 300 is a $\frac{300 - 180}{180} = 66.7\%$ increase, not a 40% change ($\frac{120}{300}$ uses the wrong base).

Strategy 3: Combination or Permutation? Ask "Does Order Matter?"

Before you pick a formula, decide whether rearranging the same items produces a different outcome.

• Handshakes, committees, choosing toppings, drawing a hand of cards ⇒ combination.
• Rankings, seat assignments, forming a number, awarding distinct prizes ⇒ permutation.

Picking the wrong one is the single most common counting error; the GRE always offers the "other" value as a distractor (e.g., 56 vs. 336 for choosing 3 of 8).

Strategy 4: Break Compound Probability into Stages

For multi-step probability, multiply the probability of each stage, adjusting the pool at each step for without replacement. For "at least one," it is usually faster to use the complement: $P(\text{at least one}) = 1 - P(\text{none}).$

Strategy 5: Use the Empirical Rule as a Ruler

When a question says "normally distributed" and gives a mean and standard deviation, translate the boundary values into a number of standard deviations and read off 68 / 95 / 99.7. Halve those figures for one-sided regions (e.g., above $\mu + \sigma$ is about $\frac{100 - 68}{2} = 16\%$).

Strategy 6: Mean Problems Are Sum Problems

Convert every "average" statement into a sum: sum $=$ mean $\times$ count. Missing-value, "what new score raises the average," and weighted-average questions all fall out of tracking the total.

Strategy 7: Multi-Select Requires Every Correct Choice

GRE "select all that apply" questions give no partial credit — you must mark every correct option and no incorrect one. Evaluate each choice independently against the exact condition rather than stopping at the first that works.