# Predicting Monthly Car Sales: The Residuals are the Story

October 7, 2014
By

(This article was first published on sweissblaug, and kindly contributed to R-bloggers)

I’ll produce predictions for US car sales by manufacture every month. There are already several blogs that describe the industry and sales that do a great job. Autoblog by the Numbers and Counting Cars are some to mention.
Unlike their analysis, I’ll try to focus on the residuals (the stuff I can’t predict) to tell the story. To highlight the difference, I think its instructive to look at what Autoblog mentioned.
The Autoblog article (link above) highlights Mitsubishi for increasing sales. However, my prediction for Mitsubishi sales are pretty much exactly what the sales were. In essence, given this model, we didn’t learn much. On the other hand, Land Rover and Jaguar had the largest residuals (in percent terms) and Land Rover and Acura had the largest deviance (Residual / Variance). I think these results are more telling because we didn’t predict them correctly; something might have changed.
I’ll publish this on a new blog: datAutomotive.
Here is a  Shiny App For Car Sales and below are graphs / tables of my current analysis and future predictions.
 Predicted Values 9/14 Actual Values 9/14 log(Predicted/Actual) Deviance Acura 488.036 576.333 0.166 1.731 Audi 632.252 621.542 -0.017 0.326 BMW 1,061.329 1,066.083 0.004 0.032 Buick 649.648 727.750 0.114 0.571 Cadillac 568.021 576.208 0.014 0.089 Chevrolet 6,284.594 6,411.375 0.020 0.135 Chrysler 1,058.392 1,199.208 0.125 0.710 Dodge 1,892.465 1,834.167 -0.031 0.172 Ford 7,374.669 7,177.542 -0.027 0.285 GMC 1,602.260 1,594.542 -0.005 0.029 Honda 4,434.780 4,349.625 -0.019 0.174 Hyundai 2,291.306 2,333.750 0.018 0.271 Infiniti 315.217 326.542 0.035 0.234 Jaguar 37.834 47.583 0.229 0.908 Jeep 2,491.378 2,301.292 -0.079 0.853 Kia 1,853.734 1,692.833 -0.091 1.115 Land.Rover 159.975 129.417 -0.212 1.816 Lexus 1,011.967 910.500 -0.106 1.189 Lincoln 285.903 302.375 0.056 0.394 Mazda 1,074.272 999.167 -0.072 0.743 Mercedes.Benz 1,186.220 1,230.125 0.036 0.364 Mini 196.975 175.792 -0.114 0.626 Mitsubishi 231.582 231.583 0.00000 0.00002 Nissan 4,176.995 3,963.250 -0.053 0.516 Porsche 156.496 150.292 -0.040 0.408 Subaru 1,727.597 1,729.875 0.001 0.026 Toyota 6,928.472 6,059.458 -0.134 1.309 Volkswagen 1,123.412 1,083.167 -0.036 0.407 Volvo 161.531 194.458 0.186 1.070

 Predicted Values for 10/14 Acura 521.878 Audi 590.125 BMW 1,051.492 Buick 666.165 Cadillac 533.468 Chevrolet 5,693.145 Chrysler 1,097.288 Dodge 1,493.426 Ford 6,694.012 GMC 1,572.525 Honda 4,049.582 Hyundai 1,998.896 Infiniti 262.475 Jaguar 40.422 Jeep 2,132.652 Kia 1,650.236 Land.Rover 160.884 Lexus 944.946 Lincoln 272.964 Mazda 894.234 Mercedes.Benz 1,184.991 Mini 183.743 Mitsubishi 193.982 Nissan 3,629.955 Porsche 158.540 Subaru 1,694.661 Toyota 5,895.316 Volkswagen 923.242 Volvo 153.691

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