## Regression and Neural Network

In this lecture, i have learn what is a linear regression, non-linear regression, logistic regression and neural network is. the lab lesson also allows me to learn how to calcuate the Rsquare, F dist, F stat.

Linear Regression

Linear regression analyzes the relationship between two variables, X and Y. For each subject (or experimental unit), you know both X and Y and you want to find the best straight line through the data. In some situations, the slope and/or intercept have a scientific meaning. In other cases, you use the linear regression line as a standard curve to find new values of X from Y, or Y from X.

The term “regression”, like many statistical terms, is used in statistics quite differently than it is used in other contexts. The method was first used to examine the relationship between the heights of fathers and sons. The two were related, of course, but the slope is less than 1.0. A tall father tended to have sons shorter than himself; a short father tended to have sons taller than himself. The height of sons regressed to the mean. The term “regression” is now used for many sorts of curve fitting.

Prism determines and graphs the best-fit linear regression line, optionally including a 95% confidence interval or 95% prediction interval bands. You may also force the line through a particular point (usually the origin), calculate residuals, calculate a runs test, or compare the slopes and intercepts of two or more regression lines.

In general, the goal of linear regression is to find the line that best predicts Y from X. Linear regression does this by finding the line that minimizes the sum of the squares of the vertical distances of the points from the line.

Note that linear regression does not test whether your data are linear (except via the runs test). It assumes that your data are linear, and finds the slope and intercept that make a straight line best fit your data.

Non-Linear Regression

Nonlinear least squares regression extends linear least squares regression for use with a much larger and more general class of functions. Almost any function that can be written in closed form can be incorporated in a nonlinear regression model. Unlike linear regression, there are very few limitations on the way parameters can be used in the functional part of a nonlinear regression model. The way in which the unknown parameters in the function are estimated, however, is conceptually the same as it is in linear least squares regression.

The biggest advantage of nonlinear least squares regression over many other techniques is the broad range of functions that can be fit. Although many scientific and engineering processes can be described well using linear models, or other relatively simple types of models, there are many other processes that are inherently nonlinear. For example, the strengthening of concrete as it cures is a nonlinear process. Research on concrete strength shows that the strength increases quickly at first and then levels off, or approaches an asymptote in mathematical terms, over time. Linear models do not describe processes that asymptote very well because for all linear functions the function value can’t increase or decrease at a declining rate as the explanatory variables go to the extremes. There are many types of nonlinear models, on the other hand, that describe the asymptotic behavior of a process well. Like the asymptotic behavior of some processes, other features of physical processes can often be expressed more easily using nonlinear models than with simpler model types.

The major cost of moving to nonlinear least squares regression from simpler modeling techniques like linear least squares is the need to use iterative optimization procedures to compute the parameter estimates. With functions that are linear in the parameters, the least squares estimates of the parameters can always be obtained analytically, while that is generally not the case with nonlinear models. The use of iterative procedures requires the user to provide starting values for the unknown parameters before the software can begin the optimization. The starting values must be reasonably close to the as yet unknown parameter estimates or the optimization procedure may not converge. Bad starting values can also cause the software to converge to a local minimum rather than the global minimum that defines the least squares estimates.

Logistic Regression

Logistic Regression is a type of predictive model that can be used when the target variable is a categorical variable with two categories – for example live/die, has disease/doesn’t have disease, purchases product/doesn’t purchase, wins race/doesn’t win, etc.  A logistic regression model does not involve decision trees and is more akin to nonlinear regression such as fitting a polynomial to a set of data values.

Logistic regression can be used only with two types of target variables:

1.  A categorical target variable that has exactly two categories (i.e., a binary or dichotomous variable).

2.  A continuous target variable that has values in the range 0.0 to 1.0 representing probability values or proportions.

As an example of logistic regression, consider a study whose goal is to model the response to a drug as a function of the dose of the drug administered.  The target (dependent) variable, Response, has a value 1 if the patient is successfully treated by the drug and 0 if the treatment is not successful.  Thus the general form of the model is:

Response = f(dose)

The input data for Response will have the value 1 if the drug is effective and 0 if the drug is not effective.  The value of Response predicted by the model represents the probability of achieving an effective outcome, P(Response=1|Dose).  As with all probability values, it is in the range 0.0 to 1.0.

One obvious question is “Why not simply use linear regression?”  In fact, many studies have done just that, but there are two significant problems:

1.  There are no limits on the values predicted by a linear regression, so the predicted response might be less than 0 or greater than 1 – clearly nonsensical as a response probability.

2.  The response usually is not a linear function of the dosage.  If a minute amount of the drug is administered, no patients will respond.  Doubling the dose to a larger but still minute amount will not yield any positive response.  But as the dosage is increases a threshold will be reached where the drug begins to become effective.  Incremental increases in the dosage above the threshold usually will elicit an increasingly positive effect.  However, eventually a saturation level is reached, and beyond that point increasing the dosage does not increase the response.

Neural Network

Neural Networks are analytic techniques modeled after the (hypothesized) processes of learning in the cognitive system and the neurological functions of the brain and capable of predicting new observations (on specific variables) from other observations (on the same or other variables) after executing a process of so-called learning from existing data. Neural Networks is one of the Data Mining techniques.

The first step is to design a specific network architecture (that includes a specific number of “layers” each consisting of a certain number of “neurons”). The size and structure of the network needs to match the nature (e.g., the formal complexity) of the investigated phenomenon. Because the latter is obviously not known very well at this early stage, this task is not easy and often involves multiple “trials and errors.” (Now, there is, however, neural network software that applies artificial intelligence techniques to aid in that tedious task and finds “the best” network architecture.)

The new network is then subjected to the process of “training.” In that phase, neurons apply an iterative process to the number of inputs (variables) to adjust the weights of the network in order to optimally predict (in traditional terms, we could say find a “fit” to) the sample data on which the “training” is performed. After the phase of learning from an existing data set, the new network is ready and it can then be used to generate predictions.

One of the major advantages of neural networks is that, theoretically, they are capable of approximating any continuous function, and thus the researcher does not need to have any hypotheses about the underlying model, or even to some extent, which variables matter. An important disadvantage, however, is that the final solution depends on the initial conditions of the network, and, as stated before, it is virtually impossible to “interpret” the solution in traditional, analytic terms, such as those used to build theories that explain phenomena.

http://www.itl.nist.gov/div898/handbook/pmd/section1/pmd142.htm

http://www.dtreg.com/logistic.htm

http://www.statsoft.com/textbook/data-mining-techniques/#neural