Marginal and conditional relationships
The effect of one explanatory variable, X, on the response, Y, can be described with a simple linear model — the marginal relationship between the variables. Alternatively a multiple regression model can be fitted containing other explanatory variables in addition to X. The coefficient of X in this model describes its conditional relationship with Y. The marginal and conditional relationships can be very different.
When interpreting the value of a coefficient in a linear model, make sure that you do so correctly in terms of a marginal or conditional relationship.
Body fat