# GMAT Indexed Graphs: Growth, Levels and Units | topin

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## Translate the index into an equation

GMAC includes interpreting graphical relationships and drawing inferences in Data Insights. The examples below are original exercises on indexed data, not claims that a particular chart appears a fixed number of times. The [Graphics Interpretation guide](/gmat/data-insights/graphics-interpretation) explains the general task and response format.

For an index defined with base-year value equal to 100, index = current value/base value × 100\. Rearranging gives current value = base value × index/100\. Read the chart’s stated definition first: the formula here applies to that particular base-100 construction, not every possible statistic called an index.

Original example: a shop sold 200 units in the base year. Its current index is 150\. Current sales are 200 × 1.5 = 300 units. The index 150 means sales are 150% of the base, or 50% higher than the base; it does not mean 150 units or 150% growth.

Attach a short note to the chart: “base year, base amount, unit”. If the amount is unknown, leave it unknown rather than treating 100 as the actual number sold. The line can still support proportional changes, but an absolute-level question may need information that the chart does not provide.

## Compare growth separately from absolute levels

__Original base-100 sales indices__
| Company | Base sales | Later index | Later sales            |
| ------- | ---------- | ----------- | ---------------------- |
| Alder   | 100 units  | 150         | 150 units              |
| Birch   | 400 units  | 120         | 480 units              |
| Cedar   | Unknown    | 160         | Unknown absolute level |

Alder’s 50% growth exceeds Birch’s 20% growth, but Birch’s later sales of 480 exceed Alder’s 150\. A higher index establishes higher proportional growth from each company’s own base here, not a higher final sales total. Cedar’s index is highest, yet its absolute total cannot be determined without its base sales.

If Cedar’s base were ten units, its later sales would be sixteen. If its base were 1,000, its later sales would be 1,600\. Both fit index 160\. Constructing these allowed cases shows why “Cedar sold the most” is unsupported, rather than merely uncertain because its number is not printed.

The distinction resembles unequal weights in [table-rate comparisons](/articles/gmat-table-analysis-weighted-averages). A relative measure hides a denominator unless it is supplied. Before using two indices as if they were comparable totals, ask whether they share a common underlying base or merely the same numerical reference value of 100.

## Calculate growth between two non-base periods

Original chart: a firm’s index rises from 120 in year two to 150 in year three, with the same base definition. Growth between those years is (150 − 120)/120 = 25%. The thirty-point index increase is not automatically 30% growth because year two is the relevant starting amount.

If base sales are 200 units, the values are 240 and 300\. The increase is 60/240 = 25%, confirming the index calculation. The actual base amount cancels when both indices share the same base, so you can calculate relative growth without knowing that amount.

For a fall from index 150 to 120, the decline is 30/150 = 20%. Reversing the direction changes the denominator. The [reverse-percentage guide](/articles/gmat-percent-change-reverse-percentages) explains this asymmetry. A graph can make the vertical movement look symmetric while the percentage changes are not.

When periods have different durations, do not automatically compare a total change with an annual growth measure. A two-year increase and a one-year increase are different intervals. Use the defined measure in the sentence, and avoid adding an annualisation rule that the question has not requested.

Original common-base comparison: both product lines have base sales of 200 units, and later indices of 130 and 160\. Their later totals are 260 and 320, so the higher index does correspond to the higher total under this equal-base condition. The lesson is not “never compare indices”; it is “identify what comparison the base permits”.

If only the first base is known, you can recover its later total but not the other’s. You may still compare each series’ growth from its own base if the index definitions match. Separate those two conclusions rather than discarding all information because one absolute amount is missing.

For a mixed chart with an indexed revenue line and an absolute staff bar, revenue per staff member needs the revenue base. Index 150 divided by 300 staff is not a currency-per-person figure. If the base revenue is £2 million, the current revenue is £3 million and revenue per staff member is £10,000\. Write the missing base as a variable when it is not supplied.

## Check mixed units and two-axis charts before dividing

Original chart description: bars show revenue in millions of pounds and a line shows staff in hundreds. A bar value of 2.4 and line value of 3 imply £2.4 million revenue and 300 staff. Revenue per member is £2,400,000/300 = £8,000\. Dividing 2.4 by 3 without unit conversion gives an uninterpreted plotted-number ratio.

The same vertical height on two axes does not imply equal values. One axis could measure millions of pounds, the other hundreds of staff. Use the legend and axis label for the selected series, then read its scale. A visually similar line crossing is not necessarily an economically meaningful equality.

If the chart shows a percentage on one axis and a total count on the other, combine them only through a defined relationship. A 20% conversion rate and 500 visitors imply 100 conversions. A 20% growth rate and 500 visitors may describe a different starting period; identify the time pairing before calculating.

Check whether labels report thousands, millions, percentages, percentage points or index values. These are not interchangeable decorations. Write the final requested unit before entering numbers into a calculator, so a precise decimal does not conceal that you used the wrong plotted quantity.

## Respect scale and measurement precision

A truncated vertical axis can exaggerate the visual size of a change without changing the labelled values. If two bars are labelled 95 and 100, the increase is 5/95, about 5.26%, even if the shorter bar looks half as tall because the displayed axis begins near 90.

For another original chart, tick marks labelled 10, 100 and 1,000 at equal spacing indicate a multiplicative scale. Equal visual steps represent a factor of ten, not an increase of ninety each time. Read labels rather than assuming every evenly spaced mark adds the same numerical amount.

Do not claim a precisely measured value when a point lies between sparse ticks and no data label is supplied. Use the answer choices and required precision. If options are widely separated, a reasonable reading may settle the comparison; if the question requires an exact value not established by the graphic, inspect additional information rather than inventing decimals.

A plotted association also does not by itself establish cause. An index line for advertising and another for sales moving together shows a pattern under their definitions. It does not prove that advertising caused the sales change. Keep graphical arithmetic separate from [argument evaluation](/articles/gmat-cr-evaluate-two-outcomes).

## Use a graph-reading checklist that changes your answer

Before solving, identify the series, base or denominator, units, interval and scale. Then restate the blank as a quantity: growth from year two, later total, revenue per staff member or difference in percentage points. This prevents you from applying a correct formula to a different plotted object.

Try three original checks. Index 80 to 100 is 25% growth, not 20%. Two companies both at index 130 have equal growth relative to their own bases, not necessarily equal totals. A revenue label of 1.8 million divided by 200 staff gives £9,000 per member. Explain each answer before checking it with arithmetic.

Review errors by what was misread. “Used index as units” and “read staff hundreds as individual people” identify distinct repairs. Change the chart labels and try a fresh example so that the habit is tested without relying on remembered numbers. For multi-part items, confirm each requested response; one sound calculation does not establish the second blank.

topin’s [free full-length GMAT mock](/gmat/practice-test), marked on the official scale, can test chart reading under a complete DI timer. Use the examples here to repair the meaning of an index, then verify transfer on unfamiliar graphics. A correct remembered answer is learning evidence, but a fresh interpretation is the stronger readiness check.

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## FAQs

What does an index of 150 mean?

Under a base-100 definition, it means 150% of the base value, or 50% above it. It does not by itself reveal an absolute amount.

Does a higher index mean higher total sales?

Not necessarily when companies use their own base values. Compare actual bases before comparing absolute totals.

What is growth from index 120 to 150?

25%, using 120 as the starting denominator. The change is thirty index points.

Can I divide numbers read from different axes directly?

Only after interpreting their units and relationship. Millions and hundreds require conversion for a per-person value.

Are equally spaced axis marks always equal additions?

No. Read the labels. A multiplicative scale can show equal visual intervals for equal ratios.

## Sources (checked 5 October 2026)

* [mba.com: official Graphics Interpretation purpose (checked 5 October 2026)](https://www.mba.com/exams/gmat-exam/about/exam-content)
* [mba.com: on-screen calculator available in DI (checked 5 October 2026)](https://support.mba.com/hc/en-us/articles/14076264362907-GMAT-Calculator-and-Scratch-Paper-Note-Taking-Policies)

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