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# Lec9 - Curve Fitting Regression and Interpolation...

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Curve Fitting Regression and Interpolation

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Lecture 9 2 Curve-Fitting: Overview z As engineers, you will need to learn to manage a variety of types of data Experimental data , e.g. PVT data, or time- dependent process data Numerical data , e.g. from a numerical solution to a differential equation, or a computer simulation of a fluid z Often you would like to fit your set of data, say a set of (x i ,y i ) points to a smooth function y=f(x) in order to capture or describe the data in a simple form z There are two major approaches based on whether the data is smooth or noisy : Noisy data -> use regression methods Smooth data -> use interpolation methods
Lecture 9 3 Statistics review z Suppose we are given a set of noisy data. z Three important quantities are the mean , the variance , and the standard deviation :

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Lecture 9 4 Regression z Regression refers to the process of trying to “fit” a smooth function through a set of noisy data z This function could be A line (“linear regression”) A higher polynomial An arbitrary function with adjustable parameters z Regression is “over- determined” in the sense that there are usually many more data points than adjustable parameters in the function
Lecture 9 5 Least-Squares Regression z There are many criteria that can be used to determine a “best fit” of a data set to a function z The most popular is the “least-squares criterion”, namely that the squared error or “residual” e i between the data points and the model, summed over all points, is as small as possible: z Notice that if the model is just a constant equal to the mean of the data set, then S r is simply related to the variance, z More generally, S r > 0 is a mean-squared error between the data and the model

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Lecture 9 6 Linear Least Squares z Linear least squares is a special case in which the model is a straight line: Two parameters: a 0 and a 1 z Criterion for choosing the parameters: S r should be minimized with respect to a 0 and a 1
7 Linear Least Squares z But since we know the data values (x i ,y i ), all the sums are just numerical coefficients. This is a 2 x 2 system of linear equations in the unknowns a 0 and a 1 that is easy to solve! z

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Lec9 - Curve Fitting Regression and Interpolation...

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