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Creating Comparison Groups from More than One Question (3 22 12)


Background and Rationale


Sometime the best design for your study will involve one target group and perhaps two or three control groups.  Extra control groups may help you to better pinpoint the specific effects of the variable you are studying.


·         In order to perform the statistical tests, you will probably need to create a new variable which includes all your groups. For example, a new variable “Diagnosis” might include -- Group1 (ADHD, but not LD), Group 2 (LD but not ADHD), and Group 3 (Neither ADHD nor LD). This new Diagnosis variable will be (or one of) the independent variable(s) for your statistical tests.

·         This can be accomplished by using the Data → Select Cases → If method.


The first step is to create your groups, randomizing and fixing group sizes as appropriate (group sizes should be approximately equal). When you have 2 -3 files containing all the groups you need for your analysis, you will need to merge them.


1. Open one of the files to be merged, then –


2. Data → Merge File → Add Cases → enter the name of one of the other files to be merged → Save As → and give this new file a name.


3. Open the new file you just created


4. Data → Merge File → Add Cases → enter the name of the last file to be merged → Save As → give this file an appropriate name, e.g. “finalDataSet”, and save it in your H drive.


You now have a data set containing all of your controlled groups for your Final project.


Creating a new variable to separate your groups

1. Create a new variable in the “Variable View” of SPSS by typing a new name in the “variable name” column, for example, “Diagnosis.”


Your data set should already be grouped up by the way you merged your files. If not:

Data → Sort Cases → Sort By → ADHD, SEX, or whatever variable groups your data appropriately in the order you need it, i.e. Group 1, Group 2, etc. Now your data will be sorted and ordered together.


2. Find your first group, e.g. the main group you are investigating - “variable1 = 1 & variable2 = 2”.


3. Enter a “1” in your “Diagnosis” variable column in the Variable View


4. Copy the “1”

- Click on the first cell and then Shift+Click the cell of the last member of Group 1, then paste the “1” into all of the columns.


Repeat this process with your other two groups, i.e. “variable1 = 2 & variable2 = 1” becomes Group 2 of “Diagnosis” and “variable1 = 2 & variable2 = 2” as Group 3. Run a Frequency Distribution to ensure your work is correct.


Now you have your final project data set with a built-in independent variable: Diagnosis.


You will probably use this new Diagnosis variable for statistical tests such as ANOVA.

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