From: Ravi Varadhan <rvaradhan_at_jhmi.edu>

Date: Mon, 11 Jun 2007 13:27:33 -0400

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https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code. Received on Mon 11 Jun 2007 - 17:45:19 GMT

Date: Mon, 11 Jun 2007 13:27:33 -0400

Spencer,

Lsoda does not "estimate" any parameters (nlmeODE does parameter estimation). It just computes the solution trajectory, at discrete times, of a dynamical systems (i.e. set of differential equations). It only works with real numbers, as far as I know.

Ravi.

Ravi Varadhan, Ph.D.

Assistant Professor, The Center on Aging and Health

Division of Geriatric Medicine and Gerontology

Johns Hopkins University

Ph: (410) 502-2619

Fax: (410) 614-9625

Email: rvaradhan_at_jhmi.edu

Webpage: http://www.jhsph.edu/agingandhealth/People/Faculty/Varadhan.html

-----Original Message-----

From: r-help-bounces_at_stat.math.ethz.ch

[mailto:r-help-bounces_at_stat.math.ethz.ch] On Behalf Of Spencer Graves
Sent: Monday, June 11, 2007 12:53 PM

To: Martin Henry H. Stevens

Cc: Jeremy Goldhaber-Fiebert; r-help_at_stat.math.ethz.ch
Subject: Re: [R] Fwd: Using odesolve to produce non-negative solutions

<in line>

Martin Henry H. Stevens wrote:

> Hi Jeremy,

*> First, setting hmax to a small number could prevent a large step, if
**> you think that is a problem. Second, however, I don't see how you can
**> get a negative population size when using the log trick.
*

SG: Can lsoda estimate complex or imaginary parameters?

> I would think that that would prevent completely any negative values

*> of N (i.e. e^-100000 > 0). Can you explain? or do you want to a void
**> that trick? The only other solver I know of is rk4 and it is not
**> recommended.
**> Hank
**> On Jun 11, 2007, at 11:46 AM, Jeremy Goldhaber-Fiebert wrote:
**>
**>> Hi Spencer,
**>>
**>> Thank you for your response. I also did not see anything on the lsoda
**>> help page which is the reason that I wrote to the list.
**>>
**>>> From your response, I am not sure if I asked my question clearly.
**>>
**>> I am modeling a group of people (in a variety of health states)
**>> moving through time (and getting infected with an infectious
**>> disease). This means that the count of the number of people in each
**>> state should be positive at all times.
**>>
**>> What appears to happen is that lsoda asks for a derivative at a given
**>> point in time t and then adjusts the state of the population.
**>> However, perhaps due to numerical instability, it occasionally lower
**>> the population count below 0 for one of the health states (perhaps
**>> because it's step size is too big or something).
**>>
**>> I have tried both the logarithm trick
*

<snip>

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https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code. Received on Mon 11 Jun 2007 - 17:45:19 GMT

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