[R] Fitting exponential curve to data points

[Andrew Clegg]

Stephen, Ted -- thanks for your input. I'm glad to know I was barking
up the right-ish tree at least.

On 7/24/07, Ted Harding <ted.harding at nessie.mcc.ac.uk> wrote:
>
> There are not enough data to properly identify the non-linearity,
> but the overall appearance of the data plot suggests to me that
> you should be considering one of the "growth curve" models.
>
> Many such models start of with an increasing rate of growth,
> which then slows down, and typically levels off to an asymptote.
> The apparent large discrepancy of your final data point could
> be compatible with this kind of behaviour.

You may have hit the nail on the head there. At least I now know that
my method would be reasonable *if* I had a genuine exponential curve.
Bound to come in handy.

> At this point, knowledge of what kind of thing is represented
> by your "count" variable might be helpful. If, for instance,
> it is the count of the numbers of individuals of a species in
> an area, then independent knowledge of growth mechanisms may
> help to narrow down the kind of model you should be tring to fit.

It's the cumulative number of citations in the MEDLINE literature
database about a particular topic (natural language processing in
biomedicine). So indeed, it can't maintain an exponential growth rate
for long, and an initial spurt while the field is novel and trendy,
followed by a levelling-off, is just what we'd expect. There was a
review a year or so ago that showed a very good exponential fit *then*
but if I could show the last point was indicative of a slowdown, that
would be news at least.

Can anyone point me at a better modelling framework than lm(), in that case?

Thanks again,

Andrew.