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

[Keld Jørn Simonsen]

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