forum.alglib.net

This is my first time using AlgLib.

I am currently using solver in excel to give me the parameters to a 5PL curve. I have a formula that I plug in the parameters to redraw a calculated curve to match the observed curve. I am not sure of the Levenberg-Marquardt is the best way of doing this but I need to be able to test the parameters without using solver. I'm not even sure how to plug in the parameters or the formula into functions. The basic formula for a 5PL curve is 1+((a-d)/(1+(x/c)^b)^m) and the parameters are A,B,C,D,M. A is lowest point in the curve, D is the highest point in the curve, B is midpoint, C is the maximum reading at infinite and m is the assymetry value.

Thank you.

Take a look at the example below. It is from development version of ALGLIB which is somewhere between 2.6 and 3.0, but it should be compatible with ALGLIB 2.6 (you may nned to change one or two function names, and that's all). The main idea is that you initialize fitting object, and then repeatedly call LSFitNonlinearIteration(), updating function value and/or gradient (depending on what is requested by algorithm).

Unfortunately, this example is not distributed with ALGLIB for VBA, because all my example are console-oriented, and VBA does not supports console, so you can't compile it (it contains calls to ConsoleOutputString() function, which is not provided by VBA).

Sergy, in the example that you sent do I need to include a formula for the degrees of freedom? Or do I just have to have it repeat the formula until it gets to the minimum?

Degrees of freedom? I don't understand it.

You have to repeatedly calculate it until it converges to minimum (while loop will end)

I guess I was confused by the comment line where you show dF/dAlpha

Still having issues. Does not seem to be giving me a change in parameters or finding a minimum. Here is my code.

Private Sub test22()
Dim x() As Double
Dim Y(7) As Double
Dim C() As Double
Dim M As Long
Dim N As Long
Dim K As Long
Dim I As Integer
Dim info As Long
Dim EpsF As Double
Dim EpsX As Double
Dim MaxIts As Double
Dim Rep As lsfit.LSFitReport
Dim State As lsfit.LSFitState

' n = number of points in the curve
' m = dimension of space points reside
' k = number of parameters


M = 8
N = 8
K = 5

ReDim x(N, M)
ReDim C(K)

Y(0) = 1.8796
Y(1) = 1.0027
Y(2) = 0.5224
Y(3) = 0.2774
Y(4) = 0.1487
Y(5) = 0.0896
Y(6) = 0.0625
Y(7) = 0.0222

x(0, 0) = 8
x(1, 0) = 4
x(2, 0) = 2
x(3, 0) = 1
x(4, 0) = 0.5
x(5, 0) = 0.25
x(6, 0) = 0.125
x(7, 0) = 0


C(0) = 0.022 'e3
C(1) = 1# 'e4
C(2) = 1.5 'e5
C(3) = 10 'e6
C(4) = 0.35 'e7

EpsF = 0
EpsX = 0.0001
MaxIts = 10

Call LSFitNonlinearFGH(x, Y, C, N, M, K, State)
Call LSFitNonlinearSetCond(State, EpsF, EpsX, MaxIts)

Do While LSFitNonlinearIteration(State)

If (State.NeedF) Then

State.F = (1 / 6.33679999999973E-04) * ((Y(0) - (((State.C(0) - State.C(3)) / (1 + (x(0, 0) / State.C(2)) ^ State.C(1)) ^ State.C(4)) + State.C(3))) ^ 2) / 3 + _
(1 / 3.12049999999964E-03) * ((Y(1) - (((State.C(0) - State.C(3)) / (1 + (x(1, 0) / State.C(2)) ^ State.C(1)) ^ State.C(4)) + State.C(3))) ^ 2) / 3 + _
(1 / 1.41120000000106E-04) * ((Y(2) - (((State.C(0) - State.C(3)) / (1 + (x(2, 0) / State.C(2)) ^ State.C(1)) ^ State.C(4)) + State.C(3))) ^ 2) / 3 + _
(1 / 1.18580000000007E-04) * ((Y(3) - (((State.C(0) - State.C(3)) / (1 + (x(3, 0) / State.C(2)) ^ State.C(1)) ^ State.C(4)) + State.C(3))) ^ 2) / 3 + _
(1 / 7.19999999999957E-53) * ((Y(4) - (((State.C(0) - State.C(3)) / (1 + (x(4, 0) / State.C(2)) ^ State.C(1)) ^ State.C(4)) + State.C(3))) ^ 2) / 3 + _
(1 / 5.61800000000029E-53) * ((Y(5) - (((State.C(0) - State.C(3)) / (1 + (x(5, 0) / State.C(2)) ^ State.C(1)) ^ State.C(4)) + State.C(3))) ^ 2) / 3 + _
(1 / 1.96020000000002E-04) * ((Y(6) - (((State.C(0) - State.C(3)) / (1 + (x(6, 0) / State.C(2)) ^ State.C(1)) ^ State.C(4)) + State.C(3))) ^ 2) / 3 + _
(1 / 4.99999999999994E-93) * ((Y(7) - (((State.C(0) - State.C(3)) / (1 + (x(7, 0) / State.C(2)) ^ State.C(1)) ^ State.C(4)) + State.C(3))) ^ 2) / 3

End If


If (State.NeedFG) Then

State.F = (1 / 6.33679999999973E-04) * ((Y(0) - (((State.C(0) - State.C(3)) / (1 + (x(0, 0) / State.C(2)) ^ State.C(1)) ^ State.C(4)) + State.C(3))) ^ 2) / 3 + _
(1 / 3.12049999999964E-03) * ((Y(1) - (((State.C(0) - State.C(3)) / (1 + (x(1, 0) / State.C(2)) ^ State.C(1)) ^ State.C(4)) + State.C(3))) ^ 2) / 3 + _
(1 / 1.41120000000106E-04) * ((Y(2) - (((State.C(0) - State.C(3)) / (1 + (x(2, 0) / State.C(2)) ^ State.C(1)) ^ State.C(4)) + State.C(3))) ^ 2) / 3 + _
(1 / 1.18580000000007E-04) * ((Y(3) - (((State.C(0) - State.C(3)) / (1 + (x(3, 0) / State.C(2)) ^ State.C(1)) ^ State.C(4)) + State.C(3))) ^ 2) / 3 + _
(1 / 7.19999999999957E-53) * ((Y(4) - (((State.C(0) - State.C(3)) / (1 + (x(4, 0) / State.C(2)) ^ State.C(1)) ^ State.C(4)) + State.C(3))) ^ 2) / 3 + _
(1 / 5.61800000000029E-53) * ((Y(5) - (((State.C(0) - State.C(3)) / (1 + (x(5, 0) / State.C(2)) ^ State.C(1)) ^ State.C(4)) + State.C(3))) ^ 2) / 3 + _
(1 / 1.96020000000002E-04) * ((Y(6) - (((State.C(0) - State.C(3)) / (1 + (x(6, 0) / State.C(2)) ^ State.C(1)) ^ State.C(4)) + State.C(3))) ^ 2) / 3 + _
(1 / 4.99999999999994E-93) * ((Y(7) - (((State.C(0) - State.C(3)) / (1 + (x(7, 0) / State.C(2)) ^ State.C(1)) ^ State.C(4)) + State.C(3))) ^ 2) / 3
End If

Loop

Call LSFitNonlinearResults(State, info, C, Rep)

Debug.Print State.C(0), State.C(1), State.C(2), State.C(3), State.C(4)
Debug.Print C(0), C(1), C(2), C(3), C(4)
End Sub

Are the values in c() supposed to be changed through the lsFit functions. Everytime I run the function the original c() values are always returned even though the function iterates 32 times I don't see any change in the values.

I see two errors in your code:
1. you don't calculate gradient value on State.NeedFG, function assumes that it is zero, that's why nothing changes
2. you should not calculate function/gradient for all points at once. On return from LSFitNonlinearIteration(State) you have point coordinates stored in State.X and function parameters stored in State.C. You should calculate function/gradient for one and only one point - which is specified by State.X - and call LSFitNonlinearIteration() again. It will cycle through points automatically