ESTIMATION OF VARIANCE IN HETEROSCEDASTIC DATA
Abstract
Data which exhibit none constant variance is considered. Smoothing procedures are applied to estimate these none constant variances. In these smoothing methods the problem is to establish how much to smooth. The choice of the smoother and the choice of the bandwidth are explored. Kernel and Spline smoothers are compared using simulated data as well as real data. Although the two seem to work very closely, Kernel smoother comes out to be slightly better.
KEY WORDS: Smoothing, Kernel, Spline, Heteroscedastic, Bandwidth, Variance. 1. Introduction
Let us have the observations .The mean is given by , . The deviations of each observation away
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Given the errors are independently distributed. However if any of these assumptions are violated the estimates obtained under the classical or usual assumption are not good. Therefore we hope to obtain better estimates of when the estimation of the variance is incorporated.
Therefore, we need to investigate and incorporate the information about the variance estimates of the errors which are needed for better understanding of the variability of the data. In heteroscedastic regression models the variance is not constant. Often as in the case with the mean, the heteroscedasity is believed to be in functional form which is referred to as variance function. We try to understand the structure of the variances as a function of the predictors such as time, height, age and so on. Two procedures of estimating the variance function includes the parametric and nonparametric methods. The parametric variance function estimation may be defined as a type of regression problem in which we see variance as a function of estimable quantities. Thus, the heteroscedasticity is modeled as a function of the regression and other structural parameters. This function is completely known, specified up to these unknown parameters. Estimation of these parameters is what entails parametric methods.
However, for many practical problems the degree to which components of the statistical model can be specified in a parametric form varies
Quality Associates, Inc., a consulting firm, advises its clients about sampling and statistical procedures that can be used to control their manufacturing processes. IN one particular application, a client game quality associates a sample of 800 observations taken during a time in which that client's process was operating satisfactorily. The sample standard deviation for there data was .21 ; hence, with so much data, the population standard deviation was assumed to be .21. Quality associates then suggested that random samples of size 30 be taken periodically to monitor the process on an ongoing basis. BY analyzing the new samples, the client could quickly learn whether the process was operating satisfactorily. when the process was not
Mean is the average of a group of scores (Woolfolk, 2014). Mean and average are used interchangeably. To find the mean, a teacher will add all of the scores together and divided by the number of tests. For example, a teacher wants to find the mean of the spelling test, the spelling test scores are as the following, 10, 8, 7, 8, 10, 10, 6, 5, 7, and 5. The first step is to add all of the scores together (76). The second step is to divided by the number of tests (10), the quotient is the mean (7.6). The first math equation is 10+8+7+8+10+10+6+5+7+5=76. The second math equation is 76/10=7.6. The mean of the
Assuming the variables used to test linear regression were continuous, had a linear relationship, had no significant outliers, showed homoscedasticity as well as independence of observations, we tested a series of bivariate regressions to explain whether or not there was a statistically significant portion of variability in the dependent variable from variability in the independent
Iterations of analysis eliminated data points that were listed as “unusual observations,” or any data point with a large standardized residual. After 5 iterations, the analysis showed improved residual plots. Randomness in the versus fits and versus order plots means that the linear regression model is appropriate for the data; a straight line in the normal probability plot illustrates the linearity of the data, and a bell shaped curve in the histogram illustrates the normality of the data.
Statistical uncertainty, which is errors due to measurement or calculation mistakes, is balanced out by the large amount of data, which is less significant comparing to the systematic uncertainty of the
Quality Associates, Inc., a consulting firm advises its clients about sampling and statistical procedures that can be used to control their manufacturing processes. In one particular application a client gave Quality Associates a sample of 800 observations taken during a time in which the client’s process was operating satisfactorily. The sample standard deviation of this data was 0.21; hence with so much data, the population standard deviation was assumed to be 0.21. Quality Associates then suggested that random samples of size 30 be taken periodically to monitor the process on an ongoing basis. By analyzing the new samples, the client could quickly learn whether the process was operating satisfactorily. When the process was not operating satisfactorily, corrective action could be taken to eliminate the problem. The design specification indicated the mean for the process should be 12. The hypothesis test suggested by Quality Associates follows.
Measure the variability to see how the data spread out in the distribution by calculating the range, mean and standard deviation of each attributes. Upon computation, the plasma glucose concentration in an oral glucose tolerance test has biggest variance which means the data of this attribute is particularly scattered.
Also, we evaluate the extent to which the samples and methods used are able to capture the random changes realized in the data obtained.
* The effect of heteroskedasticity on the OLS estimator standard errors are that the results in adjusted robust standard errors cause the homoskedasticity results to be incorrect standard errors.
The variance of the residual term should be constant. The assumption of Homoscedasticity was assessed by the plot of standardized residuals against standardized predicted values according to the recommendations of Field (2005). As we have seen in figure3 it can be assured that the point are random and evenly throughout the scattered diagram and no evidence of funnel like shape of points on one side than the other is observed, so no hetero scedasticity in the data is confirmed.
As shown in the previous chapter, the basic samples of data needed to calculate the confidence intervals have distributions which depart from the traditional parametric distributions. Thus, classical hypothesis-testing procedures based on strong parametric assumptions cannot be used to estimate the confidence intervals. In order to obtain results as reliable as possible, a statistical technique which is applicable regardless of the form of the data probability density function has to be utilized. In other words, this method should make no assumption about the different data distributions. One good candidate is the bootstrap method.
Everything is presented in general terms, allowing for any type of data covariance matrix, i.e., not only to uncorrelated observations. (-- removed HTML --) (-- removed HTML --) It is often fruitful to adopt a Bayesian view, in which the parameters of the fitting function can have a prior distribution (prior to observing the data), and from fitting, the posterior distribution is obtained. Informally stated, we have an idea about some of the parameters before observing the data (see Sec. (-- removed HTML --) I A (-- removed HTML --) for an illuminating example), and we wish to include this knowledge in our final estimate of the parameters and/or the fitted function. It is a standard procedure to incorporate such a prior distribution in linear least squares, and it can be included in the LM algorithm by, formally, treating the prior information as an additional set of data. In this work, however, it is clearly presented how the data and the prior information can be separated by exploiting the structure of the involved matrices and vectors, see Sec. (-- removed HTML --) II B (-- removed HTML --) . (-- removed HTML --) (-- removed HTML --) Unfortunately, it is not enough that models often are non-linear; even worse, they are often (not to say always) (-- removed HTML --) wrong (-- removed HTML --) . That is, whatever parameters we choose, it is impossible to reproduce the truth which is lying behind the observed data. We call this a model defect. Model defects can
• The probability distribution of the errors has constant variance • The underlying relationship between the x
There are quite number of methods of estimating regression functions, the generally used ones being the ordinary least squares (OLS) and the maximum likelihood (ML). This paper will use (OLS) over (ML) because of the properties of (OLS) that is its ability to produce best linear unbiased estimate thus