Performing Logistic Regression in PASW (SPSS)

When do we use a logistic regression?
When we want to produce odds ratios to see if our independent variables (e.g. smoking: never smoker, ex-smoker, current smoker) predicts higher odds of the dependent variable (e.g. depression: yes or no). The outcome variable must have 2 categories.

Example Scenario
Computing the odds ratio of having depression based on people's smoking behavior.

In this scenario, our dependent variable is depression, and it has 2 categories:
1=No (reference category)
2=Yes
Our independent variable is smoking behavior, and it has 3 categories:
1=Never smoked (reference category)
2=Ex-smoker
3=Current smoker

Our research question is:
Compared to those who never smoked, do those who are ex-smokers and/or those who are current smokers have higher odds of having depression?

Step 1
Analyze-> Regression-> Binary Logistic

Step 2
Select the dependent variable (depression) and move it into the Dependent box. Move the independent variable (smoke3) into the Covariates box.

Step 3
Click the Categorical... box. Move smoke3 into the categorical covariates box because smoke3 is a categorical variable (do not need this step if your independent variable is a continuous variable). Select First as the Reference Category and click change, because we want the first group (never smoked) to be the reference category.

Step 4
Click the Options... box. Tick CI for exp(B): 95% -> this will give you 95% confidence intervals for your odds ratios

As the 95%CI do not overlap, we can conclude that compared to those who have never smoked, ex-smokers have 1.14 times higher odds (95%CI=1.05 to 1.24), and current smokers have 1.79 times higher odds (95%CI=1.64 to 1.95) to be depressed.


 
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