Re: [R] problem with lm and predict - no predictions made

From: Erik Iverson <iverson_at_biostat.wisc.edu>
Date: Wed, 02 Jul 2008 20:55:54 -0500

Hello -

Keld Jørn Simonsen wrote:
> Hi
>
> I have a problem with lm and predict
>
> I have
>
> us
> [1] 2789.53 3128.43 3255.03 3536.68 3933.18 4220.25 4462.83 4739.48
> [9] 5103.75 5484.35 5803.08 5995.93 6337.75 6657.40 7072.23 7397.65
> [17] 7816.83 8304.33 8746.98 9268.43 9816.98 10127.95 10469.60 10960.75
> [25] 11685.93 12433.93 13194.70 13843.83
>
>
> us.p
> [1] 227.62 229.92 232.13 234.25 236.31 238.42 240.59 242.75 244.97 247.29
> [11] 250.05 253.39 256.78 260.15 263.33 266.46 269.58 272.82 276.02 279.20
> [21] 282.31 285.25 288.10 290.85 293.53 296.26 299.08 301.97
>
> us.l = lm(log(us) ~ log(us.p))

>> predict(us.l,n.ahead=5)

> 1 2 3 4 5 6 7 8
> 8.079754 8.131908 8.181531 8.228692 8.274111 8.320224 8.367224 8.413588
> 9 10 11 12 13 14 15 16
> 8.460813 8.509709 8.567285 8.636117 8.705057 8.772694 8.835719 8.897015
> 17 18 19 20 21 22 23 24
> 8.957402 9.019376 9.079867 9.139289 9.196752 9.250495 9.302067 9.351347
> 25 26 27 28
> 9.398927 9.446950 9.496094 9.545979
>
>
> Why does predict not give me any predictions? The result of predict() is
> same lenght (28) as the us and us.p variables.

The version of 'predict' being called on 'us.l' (i.e., predict.lm) is doing predictions, and it should be giving you a result of identical length as your original vectors. What are you expecting here? Your usage of the 'n.ahead' parameter suggests to me you might be wanting to fit your model using a different function than 'lm', and use its corresponding prediction function.



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