Friday, January 11, 2019

All measurements need context

It is a car story but it is also a good example of a statistical reality that is hard to keep in mind when reading critically. The headline is Wagon Sales Are Increasing, But They're Less Popular Than Ever by Brian Silvestro.

There is no trick here. Both statements in the headline are true. The underlying issue is that single numerical measures rarely reveal the complexity of a dynamic system. You need multiple contextual measures to comprehend what is actually happening.
Great news! Wagons sales are up. Way up, actually, versus 2013. According to data released by Edmunds and published by Bloomberg, the number of wagons sold in the US has gone up 29 percent, a total of 212,000 of them purchased in 2018. Sounds good, right? Historically, Americans have been moving away from wagons, increasingly opting for crossovers to satisfy their needs. Could this finally be the market shift that ushers in a new era of longroof prosperity?
A 29% increase in unit sales over five years. That seems indisputably a healthy harbinger for the long term production of wagons.

But of course that is not what it says.

You need to know what is happening in the whole automotive market.



If the market is growing at 50% and you are growing at 25% then your long term survival is very much in question.
From 2013 to 2018, the retail share of wagons in the market dropped from an already puny 1.9 percent to just 1.4 percent. So, while more wagons are being sold, the percentage of Americans buying wagons has actually gone down—because other types of vehicles have seen even bigger increases in the past few years. Wagons are up 29 percent in terms of raw sales numbers, but compact crossovers have gone up 75 percent in sales in the same period. It's the same story with entry-level luxury cars. Since 2013, the number sold has gone up 71 percent. And thanks to vehicles like the Chevrolet Colorado, mid-size trucks have gone up 60 percent.
Hence the truth of the headline. Unit sales are up 29% and market share is down from 1.9 to 1.4 percent. All measurements need context.

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