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The assumption in such a model is that the data distribution is independently identically distributed. Such a model has no parameters.
A third possibility is the semiparametric model . In such a model, parameters are present, but there are very weak assumptions about the distribution form of the observed data. Semiparametric models are often regarded as nonparametric and studied as such. The distinction, originating in the 1960's, is however increasingly common among statisticians. Robust procedures are required for both non- and semiparametric models because of the weak assumptions upon which they depend.
II. KAPLAN-MEIER ESTIMATOR (KME)
The Kaplan-Meier Estimator is used to calculate survival rate for subjects of a certain type. The survival rate is monitored, and the probability of continued future accessibility is calculated on a year-by-year basis. The survival rate can then be applied to larger population sizes in order to determine the likelihood of future year-by-year survival .
Such studies are complicated by subjects dying or becoming inaccessible for reasons other than those being studied. This incurs a significant amount of uncertainty, which cannot be controlled . The Kaplan-Meier procedure refers to subjects who become unavailable for such reasons as censored. They nonetheless remain part of the study in order to maintain the integrity of the global population and concurrent information.
The creators of the Kaplan-Meier estimator then determined that, while censored subjects remain as part of the study, they should be deleted from the number at risk for the next time period. They are then included once again for a later time period.
It is emphasized that the Kaplan-Meier procedure does not apply only to biological or indeed lifetime survival. It can also apply to accessibility, or the likelihood of a machine to maintain its performance for a number of years. It can also apply to the success rate of a certain action, and so on.
Because of its nature as an estimator of the survival function, one of the common applications of the Kaplan-Meier Estimator is medical science. The estimator lends itself well to this field, so that estimated survival times can be communicated to patients and family members. It can however also be applied to the shelf life of certain medicines or the success rate of certain procedures. As an estimator of these functions, the Kaplan-Meier estimator has favorable properties, including self-consistency, strong consistency, and asymptotic normality. An inherent problem is however the step-function of the estimator, necessitating a search for a smoothing function. This has led to studies that combine the kernel density estimator with the Kaplan-Meier estimator .
A recent suggestion has been the use of Bivariate survival time data in combination with the Kaplan-Meier procedure in order to smooth the data estimates. Bivariate survival time data refers to datasets where a set of two organs or limbs are estimated; such as time to visual loss in the left and right eyes, and so on. When censoring occurs, the above-mentioned nonparametric estimation becomes important for the bivariate survival function.
While the Kaplan-Meier estimator is generally univariate, the bivariate version allows for wider and smoother applications. At the time of the referenced article, there has not been a large amount of research into smooth versions of the bivariate survival function as this applies to the Kaplan-Meier estimator.
Kernel smoothing and Bezier smoothing are suggested as useful techniques for smoothing the Kaplan-Meier estimator and its data estimates. Kernel smoothing is popular and well-known in density estimation, while Bezier smoothing is popular in computational graphics, especially where this relates to computer-aided geometric design . It is shown that the density function estimator as applied via the Bezier curve has an asymptotic order, like the kernel estimator, while it also has a smaller mean squared error than the kernel estimator. Both the Bezier curve and surface are estimated to have good properties for the purpose of the investigation. These include end point interpolation, convex-hole properties, symmetry, and linear precision . Through these methods, the Kaplan-Meier estimator is smoothed and applied more favorably to medical science.
III. APPLICATION of KME to ELECTRICAL ENGINEERING -- COMMUNICATION
It does not appear that the KME has been extensively applied to the communication field. However, the possibilities for doing so are many. Particularly, the effectiveness of a given connection, as well as the security level of communication activities can be investigated by KME.
KME can for example be applied where, as seen above, data sets exist over a large conglomerate of networks. The likelihood of such data being lost as a result of their remote locations, their lack of back-up, or their inefficient storage can for example be investigated. Companies can then optimize their data in response to these estimates.
Another communication issue is connectivity. Not all service providers are equal, and the KME can be used to make a comparative study in order to determine the success rate of different providers. The cost of the different services can then be used to determine the best company to provide the service at any given time, as well as the likelihood of long-term commitment.
As mentioned above, security is also one of the most important issues in electronic communication today. There are many security systems on offer today, and the Kaplan-Meier estimator can similarly be used in a study of effectiveness for these different products.
Although traditionally used in medical science, the application possibilities of the Kaplan-Meier estimator are as numerous and potentially beneficial as those of the KDE. In today's world, electronic communication is developing so rapidly that methods such as these will be needed to estimate not only its effectiveness and likelihood of continued connectivity, but also its safety and security for users.
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