# Stepwise Regression With Diagnostic Checks

### Stepwise regression with diagnostic checks.

Regressional models are one of the most used models in real life. Maybe it’s because of the fact that they are easier to model. They are always used to show that a linear relationship exists between the variables in the datasets. Before conducting the regression analysis, you have to pool the best datasets that will assure that you make the best decision. In some cases, you might have a lot of data and want to select the explanatory variables that could lead to informed decision making. Selecting the variable that you are going to use is so critical in making a Regressional model that would lead to informed decisions. There are various ways that have been suggested by scientist that  help  in selecting the appropriate explanatory variable.

Stepwise regression.

Stepwise regression is a statistical tool that helps in the selection of the explanatory variables by using an automated procedure. When stepwise regression is applied to a set of explanatory variables, a variable is considered for addition or subtraction if it meets certain criteria. Usually, the criterion will take the form of f-test, t-test, and in some cases, the adjusted r-squared is used.With statistical tools such as SPSS the selection of the variables is automatic.  Like any other tool, if it’s used properly, it can go a long way into ensuring that a business manager makes informed decisions that will lead to the success of the business. On the contrary, if it’s not appropriately used, it could lead to a diverse effect on the business. That is why you have to know how this method works, when to use it and some of its weaknesses.

How stepwise regression works.

There are three approaches that are used in addition or selection of the variables used- forward  selection, backward selection and bidirectional elimination.Forward selection- As the names suggest, the selection here starts with n variables, and we keep adding a variable that is considered statistically significant. The process is iterated several times until the optimal result is obtained.

Backward selection- Here, we start with all the variables that we have. We then sequentially deduct each model based on the selection criterion. They exclude variables  that are considered statistically insignificant. We continue with the process until we reach an optimal result.

Bidirectional elimination. At each step, we make a decision on whether to remove or add a variable. It’s a combination of the above-mentioned selection method.

Forward selection and backward selection are the most commonly used selection approaches. But under which circumstances can you use either of the two. Use forward selection in the case when  you have a large number of variables and want to select a few of the variables. It’s like you are offtoa fishing expedition. On the other side, use backward selection when you have modest-sized data, and you want to select a few from this data.

Criticism against the stepwise regression.

Stepwise regression is a tool used by so many data scientists for data mining. However, recently because of  a lot of weaknesses, there have been some calls to stop using it. What could be some of the reasons for this?

1. Critics of stepwise regression believe that the models themselves are biased. It is not that the biases are caused by the scientist using it but it is in the data itself that is selected. They believe so because only some data is selected.
2. The model is an oversimplification of the original model.
3. Others have lamented the need for a complex computing power to develop a complex regression model.
4. There has also been an issue with the accuracy of the results provided by the models.

How can you conduct stepwise regression in SPSS?

You can easily compute the stepwise regression using SPSS. If you know how to  maneuver your way to linear regression,  the process is simple.  We have always stated, using SPSS is not some kind of rocket science, you can use it much easily  than you thought. For example, for stepwise linear regression, you can click on the Analyze tab that is visible when you open SPSS and  click on regression followed by linear regression. A dialogue box named linear regression will appear. On this dialogue box, you can select your independent and  dependent variables. To ensure that you are using stepwise regression, select stepwise in  the methods.

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