nls start values

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nls start values

Niklaus Fankhauser
I'm using nls to fit periodic gene-expression data to sine waves. I need
to set the upper and lower boundaries, because I do not want any
negative phase and amplitude solutions. This means that I have to use
the "port" algorithm. The problem is, that depending on what start value
I choose for phase, the fit works for some cases, but not for others.
In the example below, the fit works using phase=pi,  but not using
phase=0. But there are many examples which fit just fine using 0.

Is there a comparable alternative to nls that is not so extremely
influenced by the start values?

# Data for example fit
lowervals <- list(phase=0, amp=0)
uppervals <- list(phase=2*pi, amp=2)
afreq <- 1 / (24 / 2 / pi)
gene_expression <- c(1.551383, 1.671742, 1.549499, 1.694480, 1.632436,
1.471568, 1.623381,
1.579361, 1.809394, 1.753223, 1.685918, 1.754968, 1.963069, 1.820690,
1.985159, 2.205064,
2.160308, 2.120189, 2.194758, 2.165993, 2.189981, 2.098671, 2.122207,
2.012621, 1.963610,
1.884184, 1.955160, 1.801175, 1.829686, 1.773260, 1.588768, 1.563774,
1.559192)
tpoints <-
c(0,0,0,2,2,2,4,4,4,6,6,6,8,8,8,12,12,12,14,14,14,16,16,16,18,18,18,20,20,20,24,24,24)
shift=mean(gene_expression) # y-axis (expression) shift

# Perfect fit
startvals <- list(phase=pi, amp=0.5)
sine_nls <- nls(gene_expression ~ sin(tpoints * afreq + phase) * amp +
shift, start=startvals, algorithm="port", lower=lowervals, upper=uppervals)

# Convergence failure
startvals <- list(phase=0, amp=0.5)
sine_nls <- nls(gene_expression ~ sin(tpoints * afreq + phase) * amp +
shift, start=startvals, algorithm="port", lower=lowervals, upper=uppervals)

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Re: nls start values

Hans W Borchers
Niklaus Fankhauser <niklaus.fankhauser <at> cell.biol.ethz.ch> writes:

> I'm using nls to fit periodic gene-expression data to sine waves. I need
> to set the upper and lower boundaries, because I do not want any
> negative phase and amplitude solutions. This means that I have to use
> the "port" algorithm. The problem is, that depending on what start value
> I choose for phase, the fit works for some cases, but not for others.
> In the example below, the fit works using phase=pi,  but not using
> phase=0. But there are many examples which fit just fine using 0.
>
> Is there a comparable alternative to nls that is not so extremely
> influenced by the start values?
>

Use package `nls2' to first search on a grid, and then apply `nls' again
to identify the globally best point:

    # Data for example fit
    afreq <- 1 / (24 / 2 / pi)
    tpoints <- c(0,0,0,2,2,2,4,4,4,6,6,6,8,8,8,12,12,12,
                 14,14,14,16,16,16,18,18,18,20,20,20,24,24,24)
    gene_expression <-
    c(1.551383, 1.671742, 1.549499, 1.694480, 1.632436, 1.471568, 1.623381,
      1.579361, 1.809394, 1.753223, 1.685918, 1.754968, 1.963069, 1.820690,
      1.985159, 2.205064, 2.160308, 2.120189, 2.194758, 2.165993, 2.189981,
      2.098671, 2.122207, 2.012621, 1.963610, 1.884184, 1.955160, 1.801175,
      1.829686, 1.773260, 1.588768, 1.563774, 1.559192)
    shift=mean(gene_expression) # y-axis (expression) shift

    # Grid search
    library("nls2")
    frml <- gene_expression ~ sin(tpoints * afreq + phase) * amp + shift
    startdf <- data.frame(phase=c(0, 2*pi), amp = c(0, 2))
    nls2(frml, algorithm = "grid-search", start = startdf,
               control = list(maxiter=200))

    # Perfect fit
    startvals <- list(phase = 4.4880, amp = 0.2857)
    sine_nls <- nls(frml, start=startvals)
    #  phase    amp
    # 4.3964 0.2931
    # residual sum-of-squares: 0.1378

Maybe this can be done in one step.
Hans Werner

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Re: nls start values

Bert Gunter
Niklaus:

1. First, you mat not need to use nls at all, although I am not
familiar with the "port" algorithm, so I could very well be wrong
about this.  Generally speaking, one uses time series methods (e.g.
fourier analysis) to fit periodic sine waves, so you may wish to check
out CRAN's TimeSeries task view to see whether there is something
there that fits your constrained fit situation.

2. If you DO need to fit a nonlinear function the  short answer to
your questions is maybe/maybe not; obviously, Hans's suggestions may
help you get a better starting point but it still usea the same
sensitive algorithm, which is some version of gradient descent iirc.
The optimx package contains a varied collection of optimizers, some of
which may well be more robust than that of nls2. Check out that
package and the Optimization task view for background, references, and
alternatives (such as derivative-free optimizers)

Cheers,

Bert

On Tue, Dec 13, 2011 at 7:53 AM, Hans W Borchers
<[hidden email]> wrote:

> Niklaus Fankhauser <niklaus.fankhauser <at> cell.biol.ethz.ch> writes:
>
>> I'm using nls to fit periodic gene-expression data to sine waves. I need
>> to set the upper and lower boundaries, because I do not want any
>> negative phase and amplitude solutions. This means that I have to use
>> the "port" algorithm. The problem is, that depending on what start value
>> I choose for phase, the fit works for some cases, but not for others.
>> In the example below, the fit works using phase=pi,  but not using
>> phase=0. But there are many examples which fit just fine using 0.
>>
>> Is there a comparable alternative to nls that is not so extremely
>> influenced by the start values?
>>
>
> Use package `nls2' to first search on a grid, and then apply `nls' again
> to identify the globally best point:
>
>    # Data for example fit
>    afreq <- 1 / (24 / 2 / pi)
>    tpoints <- c(0,0,0,2,2,2,4,4,4,6,6,6,8,8,8,12,12,12,
>                 14,14,14,16,16,16,18,18,18,20,20,20,24,24,24)
>    gene_expression <-
>    c(1.551383, 1.671742, 1.549499, 1.694480, 1.632436, 1.471568, 1.623381,
>      1.579361, 1.809394, 1.753223, 1.685918, 1.754968, 1.963069, 1.820690,
>      1.985159, 2.205064, 2.160308, 2.120189, 2.194758, 2.165993, 2.189981,
>      2.098671, 2.122207, 2.012621, 1.963610, 1.884184, 1.955160, 1.801175,
>      1.829686, 1.773260, 1.588768, 1.563774, 1.559192)
>    shift=mean(gene_expression) # y-axis (expression) shift
>
>    # Grid search
>    library("nls2")
>    frml <- gene_expression ~ sin(tpoints * afreq + phase) * amp + shift
>    startdf <- data.frame(phase=c(0, 2*pi), amp = c(0, 2))
>    nls2(frml, algorithm = "grid-search", start = startdf,
>               control = list(maxiter=200))
>
>    # Perfect fit
>    startvals <- list(phase = 4.4880, amp = 0.2857)
>    sine_nls <- nls(frml, start=startvals)
>    #  phase    amp
>    # 4.3964 0.2931
>    # residual sum-of-squares: 0.1378
>
> Maybe this can be done in one step.
> Hans Werner
>
> ______________________________________________
> [hidden email] mailing list
> 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.



--

Bert Gunter
Genentech Nonclinical Biostatistics

Internal Contact Info:
Phone: 467-7374
Website:
http://pharmadevelopment.roche.com/index/pdb/pdb-functional-groups/pdb-biostatistics/pdb-ncb-home.htm

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Re: nls start values

Gabor Grothendieck
In reply to this post by Hans W Borchers
On Tue, Dec 13, 2011 at 10:53 AM, Hans W Borchers
<[hidden email]> wrote:

> Niklaus Fankhauser <niklaus.fankhauser <at> cell.biol.ethz.ch> writes:
>
>> I'm using nls to fit periodic gene-expression data to sine waves. I need
>> to set the upper and lower boundaries, because I do not want any
>> negative phase and amplitude solutions. This means that I have to use
>> the "port" algorithm. The problem is, that depending on what start value
>> I choose for phase, the fit works for some cases, but not for others.
>> In the example below, the fit works using phase=pi,  but not using
>> phase=0. But there are many examples which fit just fine using 0.
>>
>> Is there a comparable alternative to nls that is not so extremely
>> influenced by the start values?
>>
>
> Use package `nls2' to first search on a grid, and then apply `nls' again
> to identify the globally best point:
>
>    # Data for example fit
>    afreq <- 1 / (24 / 2 / pi)
>    tpoints <- c(0,0,0,2,2,2,4,4,4,6,6,6,8,8,8,12,12,12,
>                 14,14,14,16,16,16,18,18,18,20,20,20,24,24,24)
>    gene_expression <-
>    c(1.551383, 1.671742, 1.549499, 1.694480, 1.632436, 1.471568, 1.623381,
>      1.579361, 1.809394, 1.753223, 1.685918, 1.754968, 1.963069, 1.820690,
>      1.985159, 2.205064, 2.160308, 2.120189, 2.194758, 2.165993, 2.189981,
>      2.098671, 2.122207, 2.012621, 1.963610, 1.884184, 1.955160, 1.801175,
>      1.829686, 1.773260, 1.588768, 1.563774, 1.559192)
>    shift=mean(gene_expression) # y-axis (expression) shift
>
>    # Grid search
>    library("nls2")
>    frml <- gene_expression ~ sin(tpoints * afreq + phase) * amp + shift
>    startdf <- data.frame(phase=c(0, 2*pi), amp = c(0, 2))
>    nls2(frml, algorithm = "grid-search", start = startdf,
>               control = list(maxiter=200))
>
>    # Perfect fit
>    startvals <- list(phase = 4.4880, amp = 0.2857)
>    sine_nls <- nls(frml, start=startvals)
>    #  phase    amp
>    # 4.3964 0.2931
>    # residual sum-of-squares: 0.1378
>

Just one small point here.  If out is the result of the nls2 call
above then then coef(out) is the best set of parameter values among
those tested on the grid and:

   nls2(out, start = out)

is a quick way to run it again starting from the value found using grid search.

--
Statistics & Software Consulting
GKX Group, GKX Associates Inc.
tel: 1-877-GKX-GROUP
email: ggrothendieck at gmail.com

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Re: nls start values

Nick Fankhauser-2
In reply to this post by Hans W Borchers
Hi!

thanks a lot for this suggestion! I tried to implement it like this, and
it worked nicely.
I used the method suggested by Gabor Grothendieck for simplification:

frml <- gene_expression ~ sin(tpoints * afreq + phase) * amp + shift
gridfit <- nls2(frml, algorithm = "grid-search", data=gendat, start =
startdf)
sine_nls <- nls2(gridfit, data=gendat, start = gridfit, algorithm="port")

But I also tried the method of running the nls with start-values for
phase between 0 and 2pi in five steps and then choosing the one which
has the lowest Standardized Residual Sum of Squares. This worked even
better in some cases and I think it's also conceptually simpler.

Yours,
Nick

On 13/12/11 16:53, Hans W Borchers wrote:

> Niklaus Fankhauser <niklaus.fankhauser <at> cell.biol.ethz.ch> writes:
>
>  
>> I'm using nls to fit periodic gene-expression data to sine waves. I need
>> to set the upper and lower boundaries, because I do not want any
>> negative phase and amplitude solutions. This means that I have to use
>> the "port" algorithm. The problem is, that depending on what start value
>> I choose for phase, the fit works for some cases, but not for others.
>> In the example below, the fit works using phase=pi,  but not using
>> phase=0. But there are many examples which fit just fine using 0.
>>
>> Is there a comparable alternative to nls that is not so extremely
>> influenced by the start values?
>>
>>    
> Use package `nls2' to first search on a grid, and then apply `nls' again
> to identify the globally best point:
>
>     # Data for example fit
>     afreq <- 1 / (24 / 2 / pi)
>     tpoints <- c(0,0,0,2,2,2,4,4,4,6,6,6,8,8,8,12,12,12,
>                  14,14,14,16,16,16,18,18,18,20,20,20,24,24,24)
>     gene_expression <-
>     c(1.551383, 1.671742, 1.549499, 1.694480, 1.632436, 1.471568, 1.623381,
>       1.579361, 1.809394, 1.753223, 1.685918, 1.754968, 1.963069, 1.820690,
>       1.985159, 2.205064, 2.160308, 2.120189, 2.194758, 2.165993, 2.189981,
>       2.098671, 2.122207, 2.012621, 1.963610, 1.884184, 1.955160, 1.801175,
>       1.829686, 1.773260, 1.588768, 1.563774, 1.559192)
>     shift=mean(gene_expression) # y-axis (expression) shift
>
>     # Grid search
>     library("nls2")
>     frml <- gene_expression ~ sin(tpoints * afreq + phase) * amp + shift
>     startdf <- data.frame(phase=c(0, 2*pi), amp = c(0, 2))
>     nls2(frml, algorithm = "grid-search", start = startdf,
>                control = list(maxiter=200))
>
>     # Perfect fit
>     startvals <- list(phase = 4.4880, amp = 0.2857)
>     sine_nls <- nls(frml, start=startvals)
>     #  phase    amp
>     # 4.3964 0.2931
>     # residual sum-of-squares: 0.1378
>
> Maybe this can be done in one step.
> Hans Werner
>
> ______________________________________________
> [hidden email] mailing list
> 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.
>

______________________________________________
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