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Exercise: BMI and alcohol consumption in the 2005 questionnaire, 5

Question 5: What names are used for "23.9" and "0.074" here?
What do they tell us?
In what units are they measured?

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Suggested answer

"23.9" is called the intercept of the regression line.
It is the value BMI would have for zero alcohol consumption.
Unusually, in this case the intercept has a simple interpretation,
because zero is a real value for alcohol consumption.
We estimate that the average BMI of respondents who report zero alcohol consumption is
23.9 Kg/m^{2}.

"0.074" is called the slope or gradient of the line, also called the regression coefficient.
We estimate that the people who drink one unit of alcohol more per week than another group
will have average BMI 0.074 Kg/m^{2} greater.

The intercept is measured in the same units as the outcome variable, BMI,
so is measured in Kg/m^{2}.

The slope is measured in the units as the outcome variable per unit of the predictor variable,
units of BMI per unit of alcohol consumption,
so is measured in Kg/m^{2} per unit of alcohol per week.

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Last updated: 5 December, 2006.

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