equation y = A + Bx + e is called probabilistic model . In reality, not only one independent variable (x) affects the dependent variable (y), so an extra e is added in this equation to represent the missing or omitted variables, and random variation.
Note: 1. A and B are population parameters (or population data), while the sample parameters(or sample data) almost always are used instead of population data, the sample parameters are denoted by a and b .
2. E or e is called the random error or random error term , e represents the error for sample data.
see deterministic model