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How to Create Dummy-Coded Variables
Learning Objective: Interpret regression models with dummy-coded variables.
Learning Objective: Interpret regression models with dummy-coded variables.
How to Create Dummy-Coded Variables
Dummy-coded variables are created by only using the values of 0 and 1. The general rule used for dummy coding is that you need one (1) fewer dummy-coded variables than you have groups (# total groups – 1). So, for our variable of marital status, we would need two (2) dummy-coded variables because we have chosen to focus on three (3) marital status groups (3 – 2 = 1). The group for which we do not create a dummy-coded variable is typically called the reference category. Often the reference category will be the one that researchers want to compare to other groups. For our research, we might choose “married” as our reference category if we want to compare non-married individuals to married individuals.
Before we conduct our regression analyses in SPSS, then, we will need to create two (2) dummy-coded variables for marital status:
We will use a 1 to indicate membership to that category (e.g., to indicate that someone is divorced for the “divorced” dummy-coded variable) and 0 to indicate non-membership.
The table below shows how we would dummy-code our marital status variables.
Notice the Following
If the original value for an individual’s marital status is a 1 (indicating married), that individual would have a 0 for the “divorced” variable and a 0 for the “never married” variable. This is because they are not a “member” of either of these groups, they are not divorced, and they are not in the never-married category. This same logic holds for the remaining two (2) values of marital status. If an individual is divorced, they get a 1 for the divorced group, for example, and a 0 for the never-married group.
Also, note that each individual in the data set will have a value (either a 0 or a 1) for each dummy-coded variable that the researcher creates.
Suppose the researcher decides to add an additional marital status group (separated), so that she now has the following marital status groups: married, divorced, never married, and separated.
Hint: Count the number of groups you have and subtract 1.
How many dummy-coded variables would the researcher need to create for her regression model?
5
2
3
4
SUBMIT Incorrect TAKE AGAIN Topic 3 - Interpreting the Coefficients for Dummy-Coded Variables Caret pointing down