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Polynomial Regression in CoStatPolynomial equations have the general form: y = b0 + b1x1 + b2x2 + b3x3 + b4x4 + b5x5 + ... bnxn where b0 is an optional constant term and b1 through bn are coefficients of increasing powers of x. You must specify the order of the polynomial to which you wish to fit your data.
Data Format There must be at least two numeric columns of data; you can designate any column as the x column and any column as the y column. Rows of data with missing values in the x or y column are rejected. Options on the Statistics : Regression : Polynomial dialog box:
The Sample Run The data for the sample run is a made-up set of x and y data points: PRINT DATA 2000-08-04 16:17:44 Using: c:\cohort6\expdata.dt First Column: 1) X Last Column: 2) Y First Row: 1 Last Row: 8 X Y --------- --------- 1 2 2 3.5 3 8 4 17 5 28 6 39 7 54 8 70 For the sample run, use File : Open to open the file called expdata.dt in the cohort directory. Then:
REGRESSION: POLYNOMIAL 2002-09-26 16:11:26 Using: C:\cohort6\expdata.dt X Column: 1) X Y Column: 2) Y Degree: 2 Keep If: Calculate Constant: true Total number of data points = 8 Number of data points used = 8 Regression equation: y = 0.54464285714 -0.5625*x^1 +1.16369047619*x^2 R^2 is the coefficient of multiple determination. It is the fraction of total variation of Y which is explained by the regression: R^2=SSregression/SStotal. It ranges from 0 (no explanation of the variation) to 1 (a perfect explanation). R^2 = 0.99893689645 For each term in the ANOVA table below, if P<=0.05, that term was a significant source of Y's variation. Source SS df MS F P ------------------------ ------------- -------- --------- --------- --------- Regression 4352.83630952 2 2176.4182 2349.1054 .0000 *** x^1 4125.33482143 1 4125.3348 4452.6582 .0000 *** x^2 227.501488095 1 227.50149 245.55252 .0000 *** Error 4.63244047619 5 0.9264881 ------------------------ ------------- -------- --------- --------- --------- Total 4357.46875 7 Table of Statistics for the Regression Coefficients: Column Coef. Std Error t(Coef=0) P +/-95% CL ------------------------ --------- --------- --------- --------- --------- Intercept 0.5446429 1.342886 0.4055764 .7018 ns 3.4519984 x^1 -0.5625 0.6846597 -0.821576 .4487 ns 1.7599737 x^2 1.1636905 0.0742618 15.670116 .0000 *** 0.190896 Degrees of freedom for two-tailed t tests = 5 If P<=0.05, the coefficient is significantly different from 0. Residuals: Row X Y observed Y expected Residual --------- ------------- ------------- ------------- ------------- 1 1 2 1.14583333333 0.85416666667 2 2 3.5 4.0744047619 -0.5744047619 3 3 8 9.33035714286 -1.3303571429 4 4 17 16.9136904762 0.08630952381 5 5 28 26.8244047619 1.1755952381 6 6 39 39.0625 -0.0625 7 7 54 53.6279761905 0.37202380952 8 8 70 70.5208333333 -0.5208333333 If the constant term is not calculated (uncheck that checkbox), the curve will be forced through the origin. The results are then: REGRESSION: POLYNOMIAL 2002-09-26 16:14:38 Using: C:\cohort6\expdata.dt X Column: 1) X Y Column: 2) Y Degree: 2 Keep If: Calculate Constant: false Total number of data points = 8 Number of data points used = 8 Regression equation: y = -0.3076671035*x^1 +1.13870685889*x^2 R^2 is the coefficient of multiple determination. It is the fraction of total variation of Y which is explained by the regression: R^2=SSregression/SStotal. It ranges from 0 (no explanation of the variation) to 1 (a perfect explanation). R^2 = 0.99954387736 For each term in the ANOVA table below, if P<=0.05, that term was a significant source of Y's variation. Source SS df MS F P ------------------------ ------------- -------- --------- --------- --------- Regression 10485.4651595 2 5242.7326 6574.1784 .0000 *** x^1 9787.10294118 1 9787.1029 12272.638 .0000 *** x^2 698.362218282 1 698.36222 875.71848 .0000 *** Error 4.78484054172 6 0.7974734 ------------------------ ------------- -------- --------- --------- --------- Total 10490.25 8 Table of Statistics for the Regression Coefficients: Column Coef. Std Error t(Coef=0) P +/-95% CL ------------------------ --------- --------- --------- --------- --------- x^1 -0.307667 0.2523271 -1.219318 .2685 ns 0.6174222 x^2 1.1387069 0.0384795 29.592541 .0000 *** 0.094156 Degrees of freedom for two-tailed t tests = 6 If P<=0.05, the coefficient is significantly different from 0. Residuals: Row X Y observed Y expected Residual --------- ------------- ------------- ------------- ------------- 1 1 2 0.83103975535 1.16896024465 2 2 3.5 3.93949322848 -0.4394932285 3 3 8 9.3253604194 -1.3253604194 4 4 17 16.9886413281 0.01135867191 5 5 28 26.9293359546 1.07066404543 6 6 39 39.1474442988 -0.1474442988 7 7 54 53.6429663609 0.35703363914 8 8 70 70.4159021407 -0.4159021407 Note that the Total degrees of freedom equals the number of data points (1 greater than before), since the estimated mean was not used in the regression. The R^2 value is higher than the R^2 value for the model with a constant term(!). Remember that the R^2 value is calculated a different way when there is no constant term (see "Regression - Details - R^2" and "Regression - Constant term").
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