Simple linear regression is nothing but a regression model that estimates the relationship between one independent variable and one dependent variable using a straight line. Both variables should be quantitative.

Xi Is independent Y is dependent

EXAMPLE: Between income and happiness. Survey OF 500 people whose incomes range from $15k to $75k and ask them to rank their happiness on a scale from 1 to 10.

Here the independent variable (income) and dependent variable (happiness) are both quantitative.