Data Interpretation from Tables, Charts and Graphs
Reading the right cell
Data interpretation is a locating task before it is a calculating task. Read the title, then the units, then the row and column headings. Identify whether figures are totals or subtotals, whether they are in units, thousands or millions, and whether the table covers calendar or financial years. In charts, check the legend, the scale on each axis and whether bars are grouped or stacked. For line graphs, note whether the vertical axis starts at zero; a truncated axis exaggerates change. For pie charts, the slices show shares of a whole, so absolute values require the total.
Traps
The commonest errors are unit slips, comparing a quarterly figure with an annual one, and treating a subtotal as a category. “Increase by” differs from “increase to”, and “percentage of total” differs from “percentage change”. In stacked bars, read the segment height, not the top of the stack. Where a category is “other”, remember it is still part of the total. Check whether a percentage should be of the row total, column total or grand total.
Worked example
A table gives a firm’s quarterly revenue in thousands: Q1 240, Q2 300, Q3 270, Q4 390. Annual total = 1,200. Q4 share = 390 ÷ 1,200 = 0.325, or 32.5%. The Q2-to-Q3 change is (270 − 300) ÷ 300 = −10%, a fall of 10%, not 30.
Rule of thumb: before any calculation, state exactly what one unit on the page represents.