[R] Problems with nls

[Bert Gunter]

AFAICS this has essentially nothing to do with R. Please post elsewhere,
e.g. on a statistics list like stats.stackexchange.com.

Cheers,
Bert



On Fri, May 29, 2015 at 6:44 AM, Abolfazl Saghafi <
abolfazl.saghafi at gmail.com> wrote:

> Can some help me with a question on this bass model, please
>
> As I read some articles on this topic, I understand that
> 1. the bass formula is
> N(t) = pm + (q-p) N(t-1) - (q/m) (N(t-1))^2
> 2. which is a difference equation with the solution
> N(t) = m (1 − exp(−(p+q)t)) / (1 + (q/p)exp(−(p+q)t))
> 3. So, using a linear regression would give us some some initial
> estimations for the parameters m, p, q
> 4. we then can put the initial estimations into a NLS to get the better
> estimations
>
> Am I right?
>
> Now the question is,
> why is that I see people use cumulative data and try to fit it into a pdf
> as
> M * ( ((P+Q)^2 / P) * exp(-(P+Q) * T79) ) / (1+(Q/P)*exp(-(P+Q)*T79))^2,
>
> why not using the cumulative data and fit directly the N(t)
>
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>
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