The window changes the story
Always attach a start and end to a statement about change.
The short answer
A narrow window can answer a focused question, while a wider view supplies context. Avoid selecting a start after inspecting the chart solely to strengthen your preferred story. State the rule you used and try a neighbouring window as a sensitivity check.
A worked example
A series [100, 80, 90, 100] is flat from first to last, but rises 25% from its low to the last point. Neither statement is false. The problem arises when the chosen start is left out.
Your turn
Describe the same four-point series over points 1–4, 2–4 and 3–4. Your answers should be 0%, +25% and approximately +11.11%.
What this does not tell you
An illustrative calculation does not explain causes or predict future results. Check the dates, units and missing observations before using the method elsewhere.
Related term
Time windowSources & review
Original Charts Haven example. All stated numbers are illustrative and allow you to repeat the calculation.
Next lesson: Look beyond the window