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Dummy variable

A dummy is a nominal variable with the values 0 and 1. It can be seen as an indicator variable: applicable or not applicable.


The difference between a dichotomy and a dummy

Nominal variables are being used for making groups. If there are only two groups to be distinguished the nominal variable is a dichotomy. If the dichotomy is coded as 0 and 1, it is called a dummy.


The use of dummy variables

A dummy can be seen as an indicator variable. A 1 means applicable and 0 means not applicable. For instance a question like Did you pass the test? yes / no, is a dummy variable. A question like Do you live in ? yes / no, is a dummy variable. Likewise gender can be code: Are you male? yes / no. It’s a bit strange to say gender is a dummy variable, but you might get the idea. For me it is equal to state the question Are you female? yes / no. This is a dummy variable as well.

When in a survey a multiple response question is used, the respondent is asked to tickle aspects that are applicable for him. For instance:

Tickle the objects you have at home:

        [ ] a television
        [ ] a radio
        [ ] internet
        [ ] a washing machine
        [ ] a boiler

This isn’t one question, but in reality there are five. And all questions are a dichotomy. Even better, it delivers five dummy-variables. So you need fives columns in your database and the answers should be coded as 1 (= tickled) and 0 (= not tickled). Most programmes for online survey’s code not tickled as blank. These blanks should be replaced with zeroes because a blank means missing and that’s not the same as not tickled. I hope providers for surveys are willing to repair this error in their programme.

A dummy-variable can be used in t-tests, Mann-Whitney test and in regression. Because nominal variables that are not a dichotomy cannot be used in regression, very often such variables are transformed into a list of dummy variables. For instance the multiple choice question:

       In which country do you live?

       0 Germany
       0 French
       0 Spain
       0 Elsewhere, .....................................

can be replaced by:

       In which country do you live?

       [ ] Germany
       [ ] French
       [ ] Spain
       [ ] Elsewhere

Now you have created 4 dummy variables. Be aware not to use all four dummies in the analysis at the same time. If a person is not living in Germany, French or Spain it is obvious he lives elsewhere. So three dummy-variables are enough to take into account.


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