Regression analysis is used across business fields for tasks as diverse as systematic risk estimation, production and operations management, and statistical inference. This paper presents the cubic polynomial least square regression as a robust alternative method of making cost prediction in business rather than the usual linear regression.The study reveals that polynomial regression is a better alternative with a very high coefficient of determination. Keywords: Polynomial regression, linear regression, high-low method, cost prediction, mixed cost

Adedeji, A. Abdullahi

Isaac, O. Ajao

Ismail, I. Raji

English

International Institute for Science, Technology and Education (IISTE): E-Journals

Mathematical Theory and Modeling  www.iiste.org ISSN 2224-5804 (Paper)    ISSN 2225-0522 (Online) Vol.2, No.2, 2012  14 Polynomial Regression Model of Making Cost Prediction In Mixed Cost Analysis  Isaac, O. Ajao (Corresponding author) Department of Mathematics aand Statistics, The Federal Polytechnic, Ado-Ekiti, PMB 5351, Ado-Ekiti, Ekiti state, Nigeria. Tel: +2348035252017  E-mail: isaac_seyi@yahoo.com  Adedeji, A. Abdullahi Department of Mathematics aand Statistics, The Federal Polytechnic, Ado-Ekiti, PMB 5351, Ado-Ekiti, Ekiti state, Nigeria. Tel: +2348062632084  E-mail: anzwers2003@yahoo.com  Ismail, I. Raji Department of Mathematics aand Statistics, The Federal Polytechnic, Ado-Ekiti, PMB 5351, Ado-Ekiti, Ekiti state, Nigeria. Tel: +2348029023836  E-mail: rajimaths@yahoo.com Abstract Regression analysis is used across business fields for tasks as diverse as systematic risk estimation, production and operations management, and statistical inference. This paper presents the cubic polynomial least square regression as a robust alternative method of making cost prediction in business rather than the usual linear regression.The study reveals that polynomial regression is a better alternative with a very high coefficient of determination. Keywords: Polynomial regression, linear regression, high-low method, cost prediction, mixed cost. 1. Introduction Current practice in teaching regression analysis relies on the investigation of data sets for users with techniques that allow description and inference. There are many alternatives, however, for actual learner computation of regression coefficients and summary statistics. Kmenta (1971) presents a computational design that allows users to complete the calculations with only a pencil and paper. Brigham (1968) suggests that learners might simply construct a scatter plot and a ruler to visually approximate the regression line. Gujarati (2009) recommends the use of statistical packages which are now easily accessible to users on mainframe and micro computers (Mundrake, G.A., & Brown, B.J. (1989)).  Mixed costs have both a fixed portion and a variable portion. There are  handful of methods used by managers to break mixed costs in the two manageable components -  fixed and variable costs. The process of breaking mixed costs into fixed and variable portions allow us to use the costs to predict and plan for the future since we have a good insight on how these costs behave at various activity levels. We often call the process of separating mixed cost into fixed and variable component, cost estimation. The methods Mathematical Theory and Modeling  www.iiste.org ISSN 2224-5804 (Paper)    ISSN 2225-0522 (Online) Vol.2, No.2, 2012  15 commonly used are the Scatter graph, High-low method, and the Ordinary least square linear regression. The goal of cost estimation is to determine the amount of fixed and variable costs so that a cost equation can be used to predict future costs. 2.  Data and method The high-low method  uses the highest and the lowest activity levels over a period of time to estimate the portion of a mixed cost that is variable and portion that is fixed. Because it uses only the high and low activity levels to calculate the variable and fixed costs, it may be misleading if the high and low activity levels are not repreentative of the normal activity. The high-low method is most accurate when the high and low levels of activity are representation of the majority of the points.  Variable cost per unit (b) = 2 12 1y yx x Where 2y = the total cost at highest level of activity             1y = the total cost at lowest level of activity            2x = are the number of units at highest level of activity; and                   1x = are the number of units at highest level of activity In other words, variable cost per unit is equal to the slope of the cost level line (i.e. change in total cost / change in number of units produced).                        Total fixed cost (a) = 2 2 1 1y bx y bx    The high-low method can be quite misleading. The reason is that cost data are rarely linear and inferences are based on only two observations, either of which could be statistical anomaly or outlier. The goal of least squares is to define a line so that it fits through a set of points on a graph. Where the cummulative sum of squared distance between the points and the line is minimized, hence the name “least squares”.   2.2   Polynomial Regression model  In statistics, polynomial regression is a form of linear regression in which the relationship between the independent variable x and the dependent variable y is modeled as an nth order polynomial. Polynomial regression fits a nonlinear relationship between the value of  x and the corresponding conditional mean of y, denoted as ( )y x ( Fan, Jianqing (1996)) and (Magee, Lonnie (1998)). Although polynomial fits a non linear model to the data, as statistical estimation problem it is linear, in the sense that the regression Mathematical Theory and Modeling  www.iiste.org ISSN 2224-5804 (Paper)    ISSN 2225-0522 (Online) Vol.2, No.2, 2012  16 function  ( )y x is linear in the unknown parameters that are estimated from the data. 2.3  The model 20 1 2  +       1,2,... .i i i iy x x e i n                                                (i) Matematically a parabola is represented by the equation (i), also known as quadratic function, or more generally, a second-degree polynomial in the variable x, the highest power of of x represents the degree of the polynomial. If x3 were added to the preceeding function (Gujarati, 2009) and (Studenmund, A.H., & Cassidy, H.J. (1987)), it would be a third-degree polynomial, and so on. The stochastic version of equation (i) may be written as               2 30 1 2 3 + +      1,2,...i i i i iy x x x e i n                              (ii) Which is called a second-degree polynomial regression The general kth degree polynomial regression is written as: 20 1 2  +. . .+  +     1,2,...ki i i k i iy x x x e i n          where .error term random a is model,  theof parameters  theare , , ik1 0   3.   Data Presentation and Analysis  All  analyses were done using MINITAB 11. The scattergram in fig(i) suggests the type of regression model that will fit the data in the table above. From this figure it is clear that the relationship between total cost and output resembles the elongated S-curve. It is noticed that the total cost  curve first increases gradually and then rapidly, as predicted by the celebrated law of diminishing returns. This S-shape of the total cost curve can be captured by the following cubic or third-degree polynomial:   2 30 1 2 3 + +      1,2,...i i i i iy x x x e i n         Where y = total cost  and               x = output   3.1  Using the High-Low method Variable cost per unit (slope) =2 000 000 500 00013.04  175 000 60 000per unit,   that is N 13.04 per unit TC = FC + VC (X) Mathematical Theory and Modeling  www.iiste.org ISSN 2224-5804 (Paper)    ISSN 2225-0522 (Online) Vol.2, No.2, 2012  17 Where X = number of units Using:  Total cost (TC) = N 2 000 000 Variable cost per unit (VC) = N 13.04 and                      X = 175 000 To obtain total fixed cost (FC) N2 000 000 = FC + N 13.04 (175 000) FC =N 2 000 000 – N2 282 000 = - N 282 000. The line of best fit from the above equations becomes:                            TC =  - N 282 000 + N 13.04 (X)                        (vi)  The negative amount of fixed costs is not realistic and leads me to believe that either the total costs at either the high point or at the low point are not representative. The high low method of determining the fixed and variable portions of a mixed cost relies on only two sets of data: the costs at the highest level of activity, and the costs at the lowest level of activity. If either set of data is flawed, the calculation can result in an unreasonable, negative amount of fixed cost. It is possible that at the highest point of activity the costs were out of line from the normal relationship—referred to as an outlier.  4.   Discussion of Results  The R-Square value is a statistical calculation that characterizes how well a particular line fits a set of data. As a general rule, the closer R2 is to 1.00 the better; as this would represent a perfect fit where every point falls exactly on the resulting line. The models with the lowest P-value and highest R2 which are 0.0000895 and 0.874 are the linear and polynomial cubic regression models respectively (table 4).  The negative amount of fixed costs is not realistic and leads me to believe that either the total costs at either the high point or at the low point are not representative. The high low method of determining the fixed and    variable portions of a mixed cost relies on only two sets of data: the costs at the highest level of activity, and the costs at the lowest level of activity. If either set of data is flawed, the calculation can result in an unreasonable, negative amount of fixed cost. It is possible that at the highest point of activity the costs were out of line from the normal relationship—referred to as an outlier. All these are indications of it’s crude and unscientific nature.  5.  Conclusion and Recommendation Based on the results of the analyses it can be concluded that Polynomial regression model is  better than the conventional Linear regression and High-Low methods, especially when analysing data relating to cost and production functions. It is obvious that Linear and Quadratic models are not too bad for prediction with respect to the data used in this research paper, but the Cubic polynomial regression is better. It is therefore recommended that data Mathematical Theory and Modeling  www.iiste.org ISSN 2224-5804 (Paper)    ISSN 2225-0522 (Online) Vol.2, No.2, 2012  18 analysts should endeavour to always plot a simple scatter diagram before using any regression model in order to know the type of relationship that exists between the variable of interest.     References  Brigham, E.F. (1986). Fundamental of financial management (4th ed.). Chicago: Dryden Press. Fan, Jianqing (1996). "1.1 From linear regression to nonlinear regression". Local Polynomial Modelling and Its Applications. Monographs on Statistics and Applied Probability. Chapman & Hall/CRC   Gujarati, D.N. and Porter, D.C. (2009). Basic Econometrics. New York: McGraw-Hall. http://www.studyzone.org/testprep/math4/d/linegraph4l.cfm: Data on Monthly unit production and the associated costs Kmenta, J. (1971). Elements of econometrics. New York: Macmillan Magee, Lonnie (1998). "Non-local Behavior in Polynomial Regressions". The American Statistician (American Statistical Association) 52 (1): 20–22. Mundrake, G.A., & Brown, B.J. (1989). Applicacation of microcomputer software to university level course instruction. Journal of Education for Business, 64(3), 124-128. Stein, S.H. (1990). Understanding Regression Analysis. Journal of Education for Business, 65(6) 264-269. Studenmund, A.H., & Cassidy, H.J. (1987), Using Econometric: A practical guide. Boston: Little, Brown.       Appendix Table 1: Monthly unit production and the associated costs               (sorted from low to high) months Units (x) Cost (y) Oct 60 000 N 500 000 Nov 65 000 N 940 000 Mar 75 000 N 840 000 Sept 80 000 N 910 000 Mathematical Theory and Modeling  www.iiste.org ISSN 2224-5804 (Paper)    ISSN 2225-0522 (Online) Vol.2, No.2, 2012  19 Feb 90 000 N 1 100 000 Dec 95 000 N 1 500 000 Jan 100 000 N 1 250 000 Aug 115 000 N 1 400 000 Apr 120 000 N 1 400 000 Jun 130 000 N 1 200 000 May 140 000 N 1 500 000 Jul 175 000 N2 000 000          Fig.(i): The curve of the total cost  Mathematical Theory and Modeling  www.iiste.org ISSN 2224-5804 (Paper)    ISSN 2225-0522 (Online) Vol.2, No.2, 2012  20 16000011000060000200000015000001000000500000unitscostThe total cost curve  Table (2): Regression (Linear)   The regression equation is y = 138533 + 10.3 x                                     (iii)  Predictor       Coef       StDev          T        P Constant      138533      178518       0.78    0.456 x               10.343       1.643       6.30    0.000  S = 184068      R-Sq = 79.9%     R-Sq(adj) = 77.8%  Analysis of Variance  Source       DF          SS          MS           F        P Regression    1 1.34336E+12 1.34336E+12     39.65    0.000 Error        10 3.38811E+11  33881051933 Total        11 1.68217E+12    Mathematical Theory and Modeling  www.iiste.org ISSN 2224-5804 (Paper)    ISSN 2225-0522 (Online) Vol.2, No.2, 2012  21 Fig. (ii): Plot of the Linear regression model  160000110000 600002500000200000015000001000000 500000      0unitscostR-Sq = 0.799Y = 138533 + 10.3435X95% PI95% CIRegressionlinear regression model for total cost  Table (3):  Polynomial Regression (Quadratic)   Y = -136015 + 15.6406X - 2.33E-05X**2                      (iv) R-Sq = 0.804                            Analysis of Variance  SOURCE            DF        SS        MS         F         P Regression         2  1.35E+12  6.76E+11   18.4624  6.53E-04 Error              9  3.30E+11  3.66E+10                     Total             11  1.68E+12                                  SOURCE      DF    Seq SS         F         P Linear       1  1.34E+12   39.6492  8.95E-05 Quadratic    1  9.15E+09  0.249846  0.629176 Mathematical Theory and Modeling  www.iiste.org ISSN 2224-5804 (Paper)    ISSN 2225-0522 (Online) Vol.2, No.2, 2012  22 Fig. (iii): Plot of the Quadratic  regression model  160000110000 600002500000200000015000001000000 500000      0unitscostR-Sq = 0.804Y = -136015 + 15.6406X - 2.33E-05X**295% PI95% CIRegressionQuadratic regression model for total cost  Table (4):  Polynomial Regression (Cubic)  Y = -3888396 + 125.375X - 1.02E-03X**2 + 2.84E-09X**3          (v) R-Sq = 0.874                                           Analysis of Variance  SOURCE            DF        SS        MS         F         P Regression         3  1.47E+12  4.90E+11   18.5547  5.82E-04 Error              8  2.11E+11  2.64E+10                     Total             11  1.68E+12                                 SOURCE      DF    Seq SS         F         P Linear       1  1.34E+12   39.6492  8.95E-05 Quadratic    1  9.15E+09  0.249846  0.629176 Cubic        1  1.18E+11   4.47643  6.73E-02 Fig. (iv): Plot of the Cubic regression model Mathematical Theory and Modeling  www.iiste.org ISSN 2224-5804 (Paper)    ISSN 2225-0522 (Online) Vol.2, No.2, 2012  23 160000110000 600002500000200000015000001000000 500000      0unitscostR-Sq = 0.874Y = -3888396 + 125.375X - 1.02E-03X**2 + 2.84E-09X**395% PI95% CIRegressionCubic regression model for total cost   This academic article was published by The International Institute for Science, Technology and Education (IISTE).  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Polynomial Regression Model of Making Cost Prediction In Mixed Cost Analysis

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Polynomial Regression Model of Making Cost Prediction In Mixed Cost Analysis

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