Ask a better question
A chart is an arrangement of observations. It does not choose the question for you. Before following a line with your eyes, decide what you want to find out.
Turn a first impression into a question you can actually test.
Start with the idea
“Which line looks stronger?” is a tempting starting point, but stronger has no fixed meaning. It might mean the largest final price, the largest percentage change, the smallest fall or the steadiest path. Each question needs a different comparison. A useful question names the series, the measure and the period.
Try replacing an impression with a sentence that another reader could check: “Which series increased by the larger percentage from observation 1 to observation 4?” This question tells us exactly which two values we need for each series. It also prevents the visual height of the lines from becoming the answer.
Next, read the surrounding information. Identify the title, axis labels, unit, dates and source. A value of 120 might represent pounds, an index or a count. A line joining two points shows their order; it does not prove what happened between observations. If the chart leaves out information you need, record that gap before calculating.
A worked example
Series A starts at £100 and finishes at £120. Series B starts at £200 and finishes at £230. B finishes at the higher price and gains more pounds (£30 rather than £20). A has the larger proportional increase: 20% rather than 15%. Both statements are true because they answer different questions.
Your turn
Rewrite “B did better” as two testable questions. Which series answers each question?
Open the exercise tool →Show the answer and reasoning+
Question 1: Which series gained more pounds between observations 1 and 4? B: £230 − £200 = £30, compared with A’s £20. Question 2: Which gained more relative to its starting value? A: £20 ÷ £100 × 100 = 20%, compared with B’s 15%. Neither answer is a recommendation.
Write the question before you interpret the line.
Related term
ObservationSources & review
Original Charts Haven example. All stated numbers are illustrative and allow you to repeat the calculation.