Classroom Tips and Techniques: Least-Squares Fits
Robert J. Lopez
Emeritus Professor of Mathematics and Maple Fellow
Maplesoft
Introduction
The least-squares fitting of functions to data can be done in Maple with eleven different commands from four different packages. The CurveFitting and LinearAlgebra packages each have a LeastSquares command, the former limited to the fitting of univariate linear models; the latter, applicable to univariate or multivariate linear models. The Optimization package has the LSSolve and NLPSolve commands, the former specifically designed to solve least-squares problems; the latter, capable of minimizing a nonlinear sum-of-squares.
These seven command from the Statistics package can return some measure of regression analysis (see Table 2): Fit, LinearFit, PolynomialFit, ExponentialFit, LogarithmicFit, PowerFit, and NonlinearFit. The Fit command passes problems to either LinearFit or NonlinearFit, as appropriate. The NonlinearFit command invokes Optimization's LSSolve command, while the remaining commands (implementing a linearization where necessary) make use of LinearAlgebra's LeastSquares command.
This month's article will explore each of these eleven tools, examine the spectrum of problems to which they apply, and give examples of their use.
Tools
Table 1 summarizes the eleven least-squares commands available in Maple.
Package
Command
Comments
CurveFitting
LeastSquares
Fit a univariate linear model to data
Exact solutions supported
Fitting curve can have free parameters
LinearAlgebra
Fit a univariate or multivariate linear model to data
Obtain general or minimum-norm least-squares solution
Input can be set of linear equations
Optimization
LSSolve
Obtain local minimum of
Input: list of residuals
Supports equality and/or inequality constraints, and bounds on variables
Both supported methods use differentiation
Numeric solutions only
NLPSolve
Input:
Methods include nonlinear simplex (Nelder-Mead) for unconstrained multivariate objective functions
Statistics
Fit
Passes least-squares fit of a linear model to LinearFit, and of a nonlinear model to NonlinearFit
Accepts model only as an expression
LinearFit
Passes least-squares fit of a linear model to (numerical) LinearAlgebra
Model input as list or vector of component expressions or functions
PolynomialFit
Passes least-squares fit of a polynomial to (numerical) LinearAlgebra
Input: polynomial degree, data, and independent variable
ExponentialFit
Linearizes the fitting function to , and passes problem to (numerical) LinearAlgebra
Input: Data and independent variable
LogarithmicFit
Treats the fitting function as linear in and passes problem to (numerical) LinearAlgebra
PowerFit
NonlinearFit
Passes the least-squares fit of a nonlinear model to the LSSolve command in Optimization, obtaining a local best-fit
Input: Model as expression or function, data, independent variable
Table 1 Maple commands for least-squares fitting
The LeastSquares command in the CurveFitting package fits a univariate linear model to data. The input data can be a list of points, or separate lists (or vectors) of values for the independent and dependent variables. The data points can be weighted, and the particular linear model can be provided as an expression linear in the model parameters. Computations are done in exact/symbolic form, so the fitting curve can contain free parameters that are not solve for. Both the Context Menu for a list of lists, and the Curve Fitting Assistant provide an interactive interface to this command.
The LeastSquares command in the LinearAlgebra package provides a number of additional functionalities for linear models: the model can be multivariate; the first argument can be a set of equations; for rank-deficient models both the general and minimum-norm solutions are available; and the user has control over the name of free parameters in a general solution. Like the CurveFitting version, this command can also work in exact/symbolic mode, so inputs and outputs can contain symbolic terms.
When applied to floating-point data, the LeastSquares command in LinearAlgebra will implement calculations based either on a QR decomposition, or a singular-values decomposition. Since the QR decomposition does not readily determine the rank of the decomposed matrix, least-squares fits based on this approach can fail for rank-deficient matrices. Because the default settings for automatically switching to the more robust singular-values approach can be thwarted by a given matrix, the safest policy appears to be setting the method option to SVD in all cases.
The LSSolve command in the Optimization package provides a local solution to both linear and nonlinear least-squares problems. The objective function (a sum of squares of deviations) shown in Table 1 is minimized, possibly subject to constraints (equality, inequality, bounds on variables). The input to the command could be a list corresponding to the linear least-squares problem . Alternatively, the input to the command could be a list of residuals (deviations) of the form . If the model is given by the function , where u is a vector of parameters, then . If the least-squares solution of the (inconsistent) equations , is required, then , enabling LSSolve to accept what is essentially a list of equations.
The NLPSolve command in the Optimization package provides a local extreme for a multivariate function, whether linear or nonlinear. If this function is the sum-of-squares of deviations, then finding its minimum is equivalent to solving a least-squares problem. This command is included in Table 1 because it can invoke the Nelder-Mead method (nonlinear simplex in Maple), which is the only option in Maple that does not use differentiation to find local extrema of unconstrained multivariate functions. (Additional derivative-free options are available in Dr. Moiseev's DirectSearch package, the details of which were discussed here.)
All seven regression commands in the Statistics package work in floating-point arithmetic only. Output for each command can be one or a list of the items in Table 2, or a module containing all the relevant regression-analysis details shown in the table. The first eight items are available for the NonlinearFit command; all 16 are available for the other six regression commands. The help page for these options can be obtained by clicking here , or by executing the command ?Statistics,Regression,Solution.
degreesoffreedom
leastsquaresfunction
parametervalues
parametervector
residuals
residualmeansquare
residualstandarddeviation
residualsumofsquares
AtkinsonTstatistic
condidenceintervals
CookDstatistic
externallystandardizedresiduals
internallystandardizedresiduals
leverages
standarderrors
variancecovariancematrix
Table 2 Regression analysis elements
Table 1 indicates that the Fit command is an interface to the LinearFit and NonlinearFit commands. The LinearFit and PolynomialFit commands invoke the numeric version of the LeastSquares command in LinearAlgebra, as do the ExponentialFit, LogarithmicFit, and PowerFit commands (after linearization). The NonlinearFit command invokes the LSSolve command in Optimization.
Universe of Discourse
Tables 3 and 4 summarize the universe of discourse for the least-squares options in Maple, Table 3 dealing with univariate models; and Table 4, multivariate models. The characteristics of this universe can be taken as linear/nonlinear, overdetermined/underdetermined/exactly determined, consistent/inconsistent, univariate/multivariate, provided the notion of "determined" is given a precise meaning. At first glance, a system can have more equations than unknown parameters, but if the equations are redundant, there may actually be fewer distinct equations in the system than unknowns. Such a system would be underdetermined, but by a simple count of equations, might be called overdetermined. We opt for the former meaning, namely, that the terms underdetermined, overdetermined, and exactly determined be applied only after all redundancies have been eliminated.
According to Table 3, whether a univariate model is linear or nonlinear, if there are more distinct equations than unknowns (i.e., if the system is truly overdetermined), then the system is necessarily inconsistent, and a least-squares solution is appropriate. So too for the underdetermined, inconsistent system - a least-squares solution is appropriate, and will contain free parameters. The underdetermined, consistent system will also have a general solution containing free parameters, but here, the least-squares technique need not be invoked.
Underdetermined or exactly determined consistent systems are essentially interpolation problems; inconsistent systems are the ones requiring least-squares techniques. Underdetermined linear models will have a parameter-dependent general solution, from which can be extracted a unique solution of minimum norm ().
Univariate Models
Overdetermined
Underdetermined & Consistent
Underdetermined & Inconsistent
Linear
LeastSquares (CurveFitting)
LeastSquares (LinearAlgebra)
LSSolve (Optimization)
Fit (Statistics)
LinearFit (Statistics)
solve
LinearSolve (LinearAlgebra)
(Optimization)
Nonlinear
NonlinearFit (Statistics)
(Statistics)
(normal equations)
Table 3 Problems and tools for fitting univariate models to data
(1) Local extrema, at best. (2) Can fail for intractable algebra. (3) Minimize sum-of-squares of deviations.
Table 4 categorizes multivariate linear models, those given as , according to the number of rows (r) and columns (c) in the matrix A. However, this classification is affected by the rank of . Systems that are truly underdetermined have a parameter-dependent general solution, which, if projected onto the row space of A, becomes the minimum-norm solution. Full-rank matrices A with at least as many rows as columns have a trivial null space, and hence any associated linear system has a unique solution, even if it is in the least-squares sense.
Multivariate Linear Models
Rank of
Appropriate Commands
Full rank
Consistent (necessarily)
LinearSolve and LeastSquares (LinearAlgebra)
Deficient
Consistent
Inconsistent
Table 4 Problems and tools for fitting multivariate linear models to data
For either of Tables 3 or 4, the bifurcation induced by the exact/floating-point distinction arises only for invocations of the LeastSquares command in LinearAlgebra, and is dealt with only in the context of specific examples. Problems that turn out to be consistent and exactly determined are actually interpolation problems, and not least-squares problems.
The overdetermined nonlinear multivariate model can be solved with the NonlinearFit command in Statistics, and the LSSolve command in Optimization. Of course, the sum-of-squares of deviations can be directly minimized by, for example, the NLPSolve command in Optimization. Underdetermined nonlinear multivariate models pose a special challenge. None of the tools in LinearAlgebra apply, and the numeric tools in Optimization and Statistics provide only local solutions, so these tools will not return a general solution. If the algebra is tractable, it might be possible for the solve command to yield the general solution: for a consistent system, apply it directly to the equations; for an inconsistent system, to the normal equations.
Examples
This section contains some 21 examples illustrating the use of the Maple commands in Table 1. The organization of the examples is based on Table 3 and 4, and the remarks on nonlinear multivariate systems following Table 4.
Linear Univariate Models
Overdetermined Case
Example 1.
Fit to ten data points along .
Solution
Define , the fitting function.
Context Menu: Assign Function
Define a list of x-values (as floats).
Form a list of corresponding y-values:
Apply the LeastSquares command from the CurveFitting package.
Apply the LeastSquares command from the LinearAlgebra package.
The arguments are a set of equations of the form , and a set of parameters (the ).
Notice that the output is not the fitting function, but a set of equations defining the parameters.
To the problem in the form , apply the LeastSquares command from LinearAlgebra.
The output is now a vector of values for the parameters.
Apply the LSSolve command from the Optimization package. The input is the list .
The output is a list, the first member of which is half the sum of the squares of the deviations; and the second of which is a vector of values for the parameters.
Apply the LinearFit command from the Statistics package.
The arguments are a list of basis functions for the linear model, the data, and the independent variable for the model.
By setting infolevel to 5, additional information about the calculation is printed. The fitting function is returned.
Apply the Fit command from Statistics; the first argument must now be the model function.
Notice how Fit passed the problem off to LinearFit.
Apply the Fit command from Statistics so as to return m, a module whose exports are the entries of Table 2.
Access the exports of module singly.
=
Not recommended: Return all 16 exports in a (poorly formatted) list:
The following device line-breaks the exports, making them easier to read.
Underdetermined Case
Example 2.
Fit to the data points and .
Initialize Maple.
Define the fitting function: Context Menu: Assign Function
This is an interpolation problem requiring the solution of two consistent equations in three unknowns. It is not necessarily a least-squares problem.
Write and solve two (consistent) equations in three unknowns.
Obtain the general solution, a one-parameter family of interpolating functions.
Use the LeastSquares command from the CurveFitting package:
Although the calculation is passed off to the LeastSquares command in LinearAlgebra, which has provision for controlling the name of the free parameter, this control is lacking in the CurveFitting package.
Use the LeastSquares command from LinearAlgebra. The arguments here will be a set of equations and a set of parameters. Note the control over the free parameter.
The LeastSquares command in LinearAlgebra returns a set of equations defining the parameters, which then have to be transferred to the model to obtain the fitting function. The appearance of the free parameter is best explained via the matrix formulation of the problem.
Cast the problem in the form .
Convert equations to matrix/vector form.
Obtain the general solution.
Obtain the minimum-norm solution.
Project V onto the row space of A.
The numeric solvers of the Optimization package, being local, will not necessarily find the minimum-norm solution, and might return any member of the general solution. The numeric solvers of the Statistics package reject underdetermined problems.
Example 3.
Fit to the data point and .
Use the LeastSquares command from LinearAlgebra. The arguments here will be a set of equations and a set of parameters.
Obtain the general solution
Obtain the minimum-norm solution
The null space of A is spanned by the vectors .
Nonlinear Univariate Models
Logarithmic Model
Example 4.
Fit to the 45 data points shown in Figure 1.
X:=[.4415800447, .5203850932, .5586711668, .6765059811, .7450887166, .9557085792, .9581748469, 1.056473538, 1.566338190, 1.816703465, 2.388745916, 2.650935552, 2.961694105, 3.135006301, 3.413191570, 3.525872385, 4.033977197, 4.058984896, 4.087467046, 4.448111434, 4.478647296, 4.704561792, 4.927323253, 4.937647961, 5.212111955, 5.350970947, 6.569312236, 6.619738280, 6.636234495, 7.225453222, 7.321555106, 7.421499760, 7.628867977, 7.696458584, 7.869296084, 8.005454234, 8.192046666, 8.247384597, 8.319562068, 8.352030955, 9.377657024, 9.532776673, 9.722178164, 9.759535988, 9.887836167]:
Y:=[-.3165315447, 0.525739071e-1, .2280755754, .9930696348, 1.005519640, 2.050502588, 1.684642486, 3.030733346, 2.676977277, 5.307499018, 4.151074969, 6.894632989, 4.731555880, 7.599050973, 3.409766036, 6.936460408, 4.947406689, 6.823078626, 4.979021138, 7.125182710, 4.548574222, 9.303837054, 4.070632602, 7.469734020, 4.867068792, 8.438200908, 4.588336461, 9.204201002, 7.677634124, 10.31267885, 5.580727779, 8.814457831, 4.857491046, 10.55896517, 4.913343507, 8.240369267, 6.647593036, 9.162657236, 8.355828851, 9.204265632, 7.843490860, 9.640628922, 5.293937431, 12.36862840, 5.324349598]:
plot(X,Y,style=point);
Figure 1 Data points to be fitted with . (Data hidden behind table: lists X and Y of values of the independent and dependent variables, respectively.)
Define f, the logarithmic model function.
Form , the sum of squares of deviations:
Apply the LogarithmicFit command from Statistics
Apply the NonlinearFit command from Statistics
Minimize via the Optimization package
Form and solve the normal equations, then evaluate the resulting
The solution obtained by the linearization in LogarithmicFit closely matches the solutions obtained by methods that do not linearize.
Power Model
Example 5.
Fit to the 45 data points shown in Figure 2.
X:=[.3311845858, .3382461426, .3596315385, .4171713246, .4172203361, .4373441686, .4871624354, .5074279163, .5970838801, .6782429163, .6822631566, .6857075734, .7123173532, .7928747878, .8613884024, .9423775810, 1.013359430, 1.015837175, 1.039779012, 1.073557773, 1.087726149, 1.104538148, 1.183382262, 1.288303209, 1.371372688, 1.411264277, 1.510435824, 1.541170242, 1.721992776, 1.884733992, 1.890335165, 1.935953579, 1.943825167, 2.302765894, 2.439615410, 2.443593963, 2.554861738, 2.652786753, 2.773245229, 2.775144735, 2.868025527, 2.888689619, 2.940485400, 2.987329661, 2.996133391]:
Y:=[3.901426454, 5.552778517, 3.682748534, 4.425793308, 2.581500466, 3.925060388, 3.308707232, 4.501901432, 2.869493552, 2.887041285, 1.829623341, 3.125460247, 1.775237930, 2.823371431, 2.220192758, 2.501806865, 1.981506660, 2.571558903, 1.946127380, 2.664282284, 1.885672629, 2.425196004, 1.599871000, 1.675009728, 1.282658772, 1.885755564, .8991034242, 1.773028886, 1.093699604, 1.411716974, .8965011818, 1.511418676, 1.255943074, 1.115462934, 1.071285035, 1.070063783, .7260572291, 1.111299304, .8814155598, 1.272545739, .5739487804, 1.237321990, .8460171871, 1.022644001, .9277633116]:
Figure 2 Data points to be fitted with . (Data hidden behind table: lists X and Y of values of the independent and dependent variables, respectively.)
Apply the PowerFit command from Statistics
Evaluate for the linearized fit
The parameters computed by the linearization in PowerFit differ slightly from those computed by the other methods which don't linearize. The sum of squares of residuals returned by PowerFit is for the linearized model, not the nonlinear model; when corrected for the linearization, it is slightly larger than the value for the nonlinear fits.
Exponential Model
Example 6.
Fit to the 45 data points shown in Figure 3.
X:=[.3249060766, .4638590607, .4644519699, .4646954321, .5512620502, .5958265967, .6201785971, .6231171221, .6302313432, .6306638781, .6993610748, .7318673274, .8091876437, .9008821207, .9849002264, 1.064226572, 1.110383885, 1.259818068, 1.435261400, 1.453715259, 1.482196702, 1.532050477, 1.581696829, 1.712052277, 1.870325124, 1.872936109, 2.060662252, 2.074651185, 2.085638251, 2.114074877, 2.319376166, 2.357983108, 2.409166844, 2.417540344, 2.503005511, 2.523280179, 2.598296000, 2.603682807, 2.644623630, 2.662815612, 2.797604341, 2.896353971, 2.921562129, 2.927435786, 2.966131074]:
Y:=[1.593149583, 1.590034983, 1.011420580, 2.022496449, 1.223729582, 1.449732045, .9069637640, 1.422300135, 1.157919343, 1.543425077, 1.103220812, 1.437868212, .7945660884, 1.064526071, .9033515451, .9495079084, .8273875646, 1.159219310, .7323213934, 1.012091251, .5669217104, .8212180644, .5287792925, .6033309380, .3240342593, .5390709438, .3781516600, .6085085079, .3715978170, .5464110888, .2366337341, .3838739654, .3703637640, .4050191695, .3121357178, .3419301014, .1946629567, .3555389028, .1884514276, .4031447974, .2821896664, .3160107514, .2328625706, .2576745642, .1755520822]:
Figure 3 Data points to be fitted with . (Data hidden behind table: lists X and Y of values of the independent and dependent variables, respectively.)
Apply the ExponentialFit command from Statistics
The parameters computed by the linearization in ExponentialFit differ slightly from those computed by the other methods which don't linearize. The sum of squares of residuals returned by ExponentialFit is for the linearized model, not the nonlinear model; when corrected for the linearization, it is slightly larger than the value for the nonlinear fits.
Michaelis-Menten Model
Example 7.
Fit , the Michaelis-Menten model, to the 46 data points shown in Figure 4.
Figure 4 Data points to be fitted with . (Data hidden behind table: lists S and V of values of the independent and dependent variables, respectively.)
This example is a summary of one that appears in the ebook, Advanced Engineering Mathematics with Maple, an example that also appears in the Reporter article Nonlinear Fit, Optimization, and the DirectSearch Package. The fit is obtained with two different linearizations, and nonlinearly, the outcome being that the linearized solutions provide decidedly poor fits.
Define : Context Menu: Assign Function
Define , sum of squares of deviations.
Linearization 1
Linearization 2
The linearization requires the computation of the reciprocals and .
The linearization requires a list of ratios with new independent variable .
Obtain the least-squares regression line
Here, and .
Nonlinear Fit
Sum of Squares for :
Sum of Squares for nonlinear fitting function:
The NonlinearFit command from Statistics can return the fitting function, but the Statistics package's LSSolve command, whose input is a list of deviations (called residuals) returns the parameter values. In this example, it is even possible to form the normal equation and solve them numerically.
Figure 5 compares the graphs of the three fitting functions. Both from the graph and from the values of , it should be clear that linearizations do not necessarily provide the best fits to data.
Figure 5 Nonlinear fit (black), (red), (green) superimposed on Figure 4
Example 8.
Fit to the two points and .
This problem is essentially an interpolation, with the expected result being a one-parameter family of curves all going through the two given points. If were a linear function, such an outcome, and the means to achieve it, would be clear. For this particular , it is possible to find this one-parameter family of solutions, but in general, it might not be possible to implement the requisite manipulations.
Exact Solution
Restart Maple.
Control-drag the equation Context Menu: Assign Function
Solve for b and c as functions of a.
Obtain the one-parameter family of solutions.
The LSSolve command in the Optimization package requires at least as many residuals as parameters; otherwise, an error results. Hence, it really does not apply here.
Example 9.
The equations and would necessarily be inconsistent, so this is not an interpolation, but a problem of fitting by least squares. A general solution consisting of a two-parameter family of curves is expected.
General Solution
If necessary, define .
Define S the sum of squares.
Obtain and solve the normal equations. The parameters a and b are free, and .
Evaluate the sum of squares for this solution.
Obtain the general solution to the underdetermined, inconsistent least-squares problem.
Casual inspection shows that every member of the general solution passes through .
As in Example 8, the LSSolve command in the Optimization package requires at least as many residuals as parameters; otherwise, an error results. Hence, it really does not apply here.
Linear Multivariate Models
This section considers linear multivariate models that are cast in the form . As per Table 4, the examples are classified by the properties of the r × c matrix A, and the vector v.
Full Rank
Define the full-rank matrix
for which = .
Example 10.
Solve the least-squares problem , where , and .
Since there are fewer equations than variables, the system cannot be inconsistent. Each row in A represents the left-hand side of a distinct equation, so no matter what appears on the right, the equations in the system must be consistent.
Apply the LeastSquares command, designating s as the free variable base-name. The result is the general solution containing one free parameter. The solution is u, a vector of parameter values.
Obtain the minimum-norm least-squares solution. This is the projection of the general solution onto the row space of Af.
Obtain P, the matrix that projects onto the row space of A.
The columns of N are a basis for the row space.
Project onto the row space. The result is the minimum-norm solution, a vector that lies in the row space of Af.
Rank-Deficient
Define the rank-deficient matrix
Example 11.
That the system is consistent can be seen from
General solution:
Minimum-norm solution:
Example 12.
That the system is inconsistent can be seen from
A full-rank matrix in a square system must necessarily be consistent, and therefore have a unique solution. There cannot be a least-squares problem in this case.
Example 13.
Because this system is essentially just an underdetermined one, a general solution is available with the LinearSolve command in the LinearAlgebra package.
Seek a least-squares solution numerically:
That this is a member of the general solution can be seen by projecting it onto the row space of Bd.
Project onto the row space. The result is the minimum-norm solution, a vector that lies in the row space of Bd.
However, to obtain the minimum-norm solution numerically, specify that the calculation is to be based on the singular value decomposition, rather than on the default QR decomposition.
Example 14.
The default method, based on a QR decomposition, utterly fails because this decomposition does not have an efficient way to determine rank. For problems such as this (and Example 13), specify the method as the one based on the singular value decomposition.
Alternatively, use the LinearFit command from the Statistics package. Although this command is based on the LeastSquares from LinearAlgebra, there is an additional wrapper that attempts to deal with the issues raised by numeric calculations.
The deficiency in rank of the matrix has been detected, and the calculation is based on the singular value decomposition. The control is via the ratio of the smallest to the largest singular values, which is the reciprocal of an estimated condition number for the input matrix. If this ratio is smaller than the default threshold , the matrix is deemed to be ill-conditioned, and the least-squares calculation is based on the singular value decomposition. This default threshold is modified with the svdtolerance parameter.
The reciprocal of the estimated condition number is slightly larger than , but that is well below the default threshold of , so the first least-squares calculation is based on the singular value decomposition; in the second where the reciprocal of the estimated condition number is slightly larger than the threshold, the calculation is based on the default QR decomposition, and consequently fails.
Example 15.
Consequently, this is not a least-squares problem, but a properly determined system with a unique solution, obtainable for example, by LinearSolve in LinearAlgebra.
Example 16.
Because the matrix is full-rank, the null space is empty, and the least-squares solution is unique.
In general, the sum-of-squares of residuals is given by .
Alternatively, the solution can also be found with the LSSolve command in the Optimization package.
The first member of the output list is half the sum-of-squares of the residuals; doubling this number gives = .
Example 17.
The general solution of an overdetermined but consistent system can also be found with the LinearSolve command from LinearAlgebra.
It is left to the reader to show that by appropriately redefining the free parameter in one general solution, the other will be obtained.
Example 18.
Numeric linear algebra:
This calculation fails because the default QR-based method does not recognize that the rank-deficiency of the matrix. The more robust SVD-based method must be invoked.
The LSSolve command from the Optimization package can find only a local solution, that is, one member of the general solution family.
Project this solution onto the row space of
The columns of N are a basis for the row space; P projects onto the row space.
The projection is the minimum-norm solution.
In this example, the LinearFit command from the Statistics package finds the minimum-norm solution, but this outcome is dependent on the relative values of the default setting of the svdtolerance parameter, and the reciprocal of the approximate condition number computed for .
The rank-deficiency of had been detected, and the SVD-based method invoked. The minimum-norm solution is returned.
The reciprocal of the approximate condition number of :
This value is smaller than , the default svdtolerance parameter, so the more robust SVD-based method is invoked.
Nonlinear Multivariate Fit
The first two columns of the matrix
are the abscissas and ordinates, respectively, of five data points . The numbers in the third column are five corresponding observations .
Example 19.
Fit the function to the data in M.
Since = , these data points generate a set of overdetermined nonlinear equations that are necessarily inconsistent. In contrast to the linear case, there is no functionality for obtaining a least-squares fit for nonlinear equations. The tools of Statistics and Optimization are the only ones that apply.
Specify the nonlinear model: define the function .
Form , the sum of squares of residuals.
Apply the LSSolve command from Optimization
Half the sum of squares is given by . Double it to get the minimized .
Apply the Minimize command from Optimization
The results from all three approaches are fairly consistent.
are the abscissas and ordinates, respectively, of two data points . The numbers in the third column are two corresponding observations .
Example 20.
Since the data generate a set of two equations in three unknown parameters, this is an interpolation problem in which = suggests there will be a general solution with one free parameter. In the nonlinear case, there is no theory by which a (unique) minimum-norm solution is extracted.
Specify the nonlinear model by defining .
From the two given data points, form two equations in the three unknown parameters.
Solve two equations for any two parameters in terms of the third. Here, is the free parameter.
The general solution is a fitting function dependent on one free parameter:
Numeric solutions that seek to minimize a sum-of-squares of residuals return, at best, individual members of this family of solutions.
Example 21.
The data determines two inconsistent equations in the three unknown parameters . This is no longer an interpolation; it is a least-squares problem.
Define P and Q, the two data points.
Form SS, the sum of squares of residuals.
Form and solve the three normal equations.
The general solution is a fitting function dependent on two free parameters:
Numeric solutions that seek to minimize return, at best, individual members of this family of solutions.
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