CURVE ESTIMATION AND ESTIMATOR PROPERTIES OF THE NONPARAMETRIC REGRESSION TRUNCATED SPLINE WITH A MATRIX APPROACH

Regression analysis is one of the statistical analyses used to estimate the relationship between the predictor and the response variable. Data are given in pairs, and the relationship between the predictor and the response variable was assumed to follow a nonparametric regression model. This model is flexible in estimating the curve when a typical data pattern does not follow a specific pattern. The nonparametric regression curve was approached by using the truncated spline function with several knots. The truncated spline estimator in nonparametric regression is linear in the observation. It is highly dependent on the knot points. The regression model's random error is assumed to have an independent normal distribution with zero mean and equal variance. The truncated spline's curve estimate was obtained by minimizing the error model through the least squared optimization method. The nonparametric regression truncated spline's estimator properties are linear, unbiased, and if the error is normally distributed, the estimator is normally distributed.


INTRODUCTION
Nonparametric regression has received a lot of attention from researchers. This approach loosens assumptions about linearity, as well as information about functional form in regression analysis. Besides, this approach allows data to be explored more flexibly. It is a flexible regression in estimating the regression curve when, in some circumstances, a common data pattern does not follow a specific pattern (Mahmoud, 2019).
There are several approaches to nonparametric regression, one of which is the truncated spline approach. Among the nonparametric regression models, spline regression has several features, such as it is a model with particular and excellent statistical interpretation and visual interpretation. The spline can model data on changing data patterns at certain sub-intervals because the spline is a polynomial slice with segmented properties. This segmented property provides more flexibility than ordinary polynomials, making it possible to adapt more effectively to the local characteristics of a function or data (Budiantara et al. (2015); Nurcahayani et al., (2019); Wening et al. (2020)).
Previous research has been carried out by applying truncated spline nonparametric regression to various case data, such as pattern data related to poverty, population, education, and health (Budiantara et al. (2012) Nurcahayani et al. (2019)). This study is conducted related to the curve estimation and estimator properties of one of the nonparametric regression models with a matrix approach. It is expected to provide a scientific insight into the process of curve estimation, and the properties of the nonparametric regression model's estimators with the spline truncated approach.

CURVE ESTIMATION OF THE NONPARAMETRIC REGRESSION TRUNCATED SPLINE
The spline function is the sum of the polynomial functions with a truncated function.
Consider the nonparametric regression model, where the curve estimation is done using a spline. Given paired data ( , ) and the relationship between and assumed to follow a nonparametric regression model: This study is carried out with the regression curve approached with the spline function with knots . The spline function approaches regression curve with points of knot , the spline regression model is obtained according to equation (1). If equation (1) is presented in a matrix form, the following form is obtained.
The shape of the above equation's regression curve ( ) is assumed to be unknown, while the errors , = 1,2, ⋯ , are mutually independent with zero mean and 2 variance. The spline function in equation (2) can be described in the following form: where and are a real constant, and a truncated function as in equation (3). If the truncated spline regression model is presented in the form of a matrix, it is obtained: or it can be written as: Furthermore, the parameter estimates of are obtained using the least square method by completing the optimization.
In a more straightforward form of presentation, the estimator, can be obtained by solving an optimization that minimizes the number of squares of error concerning the vector . Then the result is equalized to zero.

ESTIMATOR PROPERTIES OF THE NONPARAMETRIC REGRESSION TRUNCATED SPLINE
Further study is conducted regarding the properties of the truncated spline estimator in nonparametric regression.

a. Linear Estimator
In this section, a study of the linearity of the spline estimator in nonparametric regression is conducted.
The spline nonparametric regression model can be written in the following matrix form with = ( 1 , 2 , . . . , ).

= [ ] +
If [ ] is written by , with [ ] is the function matrix of , we get: Based on the above equation, it can be seen that the spline estimator ̂ is linear. This linearity can make it easier for researchers to form inference statistics for the spline approach.

b. Unbiased Estimator
In this section, the expected value of the estimator spline ̂ is determined to see whether the estimator is biased or not. Then obtained, indicates that the estimator ̂ is unbiased.

c. Normally Distributed Estimator
In this section, a study is conducted regarding the distribution of the spline estimator in nonparametric regression. In inference statistics, random error is assumed to follow a normal distribution with zero mean , ( ) = = , and 2 variance. Since the spline nonparametric regression model's error is normally distributed, written as ( , 2 ), the Moment Generating Function (MGF) of the vector is given as follows.  Furthermore, taking into account the equation ̂= [ ] , we will look for the distribution of the estimator ̂. By using MGF, the following results were obtained.
In the previous section, we got ( ) =

CONCLUSIONS
From the studies that have been done, it can be seen that the spline estimator in nonparametric regression is linear in the observation = ( 1 2 ⋯ ) and is highly dependent on the knot points 1 , 2 , . . . , . The curve estimate for the truncated spline was obtained, namely ̂( ) = [ ] , with [ ] indicates the function of knot points = ( 1 , 2 , . . . , ) . The estimator's properties are linear, unbiased, and if the error is normally distributed, the estimator is normally distributed.