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

From: Keld Jørn Simonsen <keld_at_dkuug.dk>
Date: Thu, 03 Jul 2008 09:41:17 +0200

On Wed, Jul 02, 2008 at 08:55:54PM -0500, Erik Iverson wrote:
> 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.

Yes, what I have is actually some time series since 1980 on various countries - here USA production and population. I would like to estimate a model and then extrapolate for future years. Maybe predict.lm() is not the right function for that. What would be an adequate function then?

best regards
keld



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