Chapter 9. Changes Over Time

“Perfection is attained by slow degrees; it requires the hand of time.”

Voltaire

When it comes to communicating data, it’s all about time. Time is the one factor that simply can’t be removed from the equation. Look at any presentation of data on the Web, in print, or at the office. You won’t have to look very hard to see the element of time. It’s usually right on the surface: monthly unemployment rate, quarterly earnings per share, top-grossing movies of the weekend. If the element of time isn’t explicitly stated somewhere, it probably should be.

So far, we’ve compared quantities summed up over a period of time, like total points in a player’s career or tons of trash collected in New York City during the month of September. While time was a factor, we didn’t consider how these quantities varied over the course of time, other than control charts in Chapter 7. We simply defined the start and stop points, and lumped everything between into one bucket.

But we can unlock a whole new dimension of insight if we consider how the measurable universe has changed, is changing, or likely will change with time:

  • Is the situation getting better or worse, or continuing the same?

  • Has time treated one variable differently than another?

  • Are there trends, outliers, or shifts in the data over a specific period of time?

  • Is the degree of variation over time significant or just mere noise?

These are all questions that are central to communicating data with respect to time. There are ...

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