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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