A researcher had a categorical variable that they wanted to include as a predict

Question

A researcher had a categorical variable that they wanted to include as a predictor in a regression equation. The researcher was trying to predict the success of a back pain intervention, and the categorical variable was the duration of the back pain prior to treatment, with 4 categories: less than 6 months, 6–12 months, 1–2 years, more than 2 years. They needed to code these variables into dummy variables for the regression using less than 6 months as their reference category.

How should the correct coding scheme be represented in terms ofDummy Variables 1, under 6 months vs 6-12 months, 2 under 6 months vs 1-2 years and 3 under 6 months vs over 2years?

Explanation / Answer

we need (n-1) dummy variable .where n is the number of levels of the categorical variable.

here n = 4 , hence 3 dummy variable

less than 6 months, - a

6–12 months - b

1–2 years, - c

a = 1 if less than 6 months else 0

b = 1 if 6-12 months else 0

c = 1 if 1-2 years else 0

if a = b =c = 0 then automatically it is over 2 years

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