GMAT Focus Edition — Data Insights: Table Analysis · Graphics Interpretation · Multi-Source Reasoning · Two-Part Analysis
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Data Insights Lesson 13 of 20

Lesson 13:
Bar & Line Charts — Visual Delta Logic

Compute period-over-period changes, identify delta patterns, and distinguish level from rate using bar and line chart combinations.

⏱ 50 mins
🎯 DI 75 to 88
📚 Prereq: Lessons 6–7
Note: Lesson 13 of 20 — Bar & Line Charts — Visual Delta Logic.
1

Core Concepts: Bar & Line Charts — Visual Delta Logic

Compute period-over-period changes, identify delta patterns, and distinguish level from rate using bar and line chart combinations.

Key Framework
Delta = New − Old
Change between two periods. Positive = growth. Negative = decline.
% change = Delta / Old × 100
Always use the earlier value as the denominator.
Largest delta ≠ largest % change
Compare rates, not absolute differences, when base values differ.
Combined chart: check which axis is which
Two y-axes serve different series. Always match series to axis before computing.
2

Application Strategy

Step-by-Step Approach
①
Identify the question type and the specific metric being asked about.
②
Locate the relevant data in the chart, table, or tab.
③
Apply the key formula or logic rule from the framework above.
④
Verify that your answer satisfies the question as stated.
3

Visual Reference Diagram

Visual Framework for Lesson 13
📊
Delta = New − Old
📈
% change = Delta / Old × 100
🔍
Largest delta ≠ largest % change
📋
Combined chart: check which axis is which
4

Quick Reference Rules

▸
Delta = New − Old: Change between two periods. Positive = growth. Negative = decline.
▸
% change = Delta / Old × 100: Always use the earlier value as the denominator.
▸
Largest delta ≠ largest % change: Compare rates, not absolute differences, when base values differ.
▸
Combined chart: check which axis is which: Two y-axes serve different series. Always match series to axis before computing.
5

10 Traps for Lesson 13

⚠ Confusing the key formula for this topic

Review the core rule: Delta = New − Old

⚠ Reading the wrong data series

Always verify axis labels and legends before extracting values.

⚠ Ignoring units or scale

Check units on every axis before computing ratios or changes.

⚠ Treating estimates as exact

GI data requires interpolation — use approximate values and pick the closest answer.

⚠ Forgetting the denominator

% change = Delta / Old × 100 — always identify what you're dividing by.

⚠ Applying simple average when weighted is needed

If group sizes differ, compute weighted average, not simple mean.

⚠ "All" statements need every row verified

One counterexample makes a universal statement False.

⚠ Scope: data sample ≠ full population

Conclusions are limited to the data range provided, not broader populations.

⚠ Confusing absolute and relative measures

Absolute change ($) and relative change (%) answer different questions.

⚠ Not using the complement when helpful

P(at least one) = 1 − P(none). In set logic, use complement for "not" conditions.

✦

10 Practice Questions

Q1 of 10
GI~550

A bar chart shows Jan revenue $120K and Feb revenue $150K. The month-over-month change is:

Explanation: $30K increase. Delta = $150K − $120K = $30K. The % change = 30/120 = 25%.
Q2 of 10
GI~600

Revenue grew from $200M (Year 1) to $260M (Year 3). Two-year percentage change is:

Explanation: 30%. (260−200)/200 = 60/200 = 30%.
Q3 of 10
GI~650

A line graph shows that the largest period-over-period decline occurred when revenue fell from $500K to $350K. This decline as a percentage is:

Explanation: 30%. (500−350)/500 = 150/500 = 30%.
Q4 of 10
GI~600

A dual-axis chart shows bars (left axis: sales volume) and a line (right axis: price per unit). In Q3: volume bar = 800 units, price line = $75. Revenue in Q3 is:

Explanation: $60,000. Revenue = volume × price = 800 × $75 = $60,000.
Q5 of 10
GI~550

A bar chart shows growth across 5 years. Year 1 to 2: +$10M. Year 2 to 3: +$15M. Year 3 to 4: +$8M. Year 4 to 5: +$20M. The year with the largest absolute increase is:

Explanation: Year 4 to 5 (+$20M). The largest single-period absolute increase was +$20M in Year 4 to 5.
Q6 of 10
GI~650

A line graph shows Unit Cost declining from $80 to $50 over 4 years. The percentage decline is:

Explanation: 37.5%. (80−50)/80 = 30/80 = 37.5%.
Q7 of 10
GI~550

A bar chart has bars of height 40, 60, 50, 70, 45 for months Jan-May. The month with the second-largest value is:

Explanation: April (70 is largest, 60 is second-largest = February). Sorted: 70, 60, 50, 45, 40. Second-largest = 60 = February.
Q8 of 10
GI~700

Revenue in Q1 was $300M. It grew 20% in Q2 and fell 10% in Q3. Revenue in Q3 is:

Explanation: $324M. Q2 = 300 × 1.20 = $360M. Q3 = 360 × 0.90 = $324M.
Q9 of 10
GI~700

A bar chart shows 2022 and 2023 data for 5 departments. Which department had the highest percentage change (any direction)?

Explanation: Highest |% change| = highest |(New−Old)/Old|. You must compute the percentage change for each department, not just look at absolute bar heights. The department with the smallest base that changed even a little can have the highest % change.
Q10 of 10
TA~700

Is the following True/False? "A 50% increase followed by a 50% decrease returns to the original value."

Explanation: False — 25% net decrease. Start at 100. After 50% increase: 150. After 50% decrease from 150: 150×0.5 = 75. Net = 75 − 100 = −25. A 50% gain followed by a 50% loss is NOT zero-sum because the base changes.
Lesson Summary
Delta = New − Old

Change between two periods. Positive = growth. Negative = decline.

% change = Delta / Old × 100

Always use the earlier value as the denominator.

Largest delta ≠ largest % change

Compare rates, not absolute differences, when base values differ.

Combined chart: check which axis is which

Two y-axes serve different series. Always match series to axis before computing.

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