Use of the regression line

Use of the regression line

Understanding the Regression Line

  • The regression line, also known as the line of best fit, is a tool used in statistics to predict the value of one variable given the value of another.
  • It represents an equation, usually of the form y = a + bx where ‘a’ is the y-intercept, ‘b’ is the slope of the line, ‘x’ is the independent variable, and ‘y’ is the dependent variable.
  • The regression line is drawn in such a way that the sum of the squared residuals (the difference between actual and predicted values) is minimised, hence it provides the best assessment of the relationship between the two variables.

Using the Regression Line for Predictions

  • The regression line can be used to predict the value of the dependent variable (y) for a given value of the independent variable (x).
  • However, the prediction should only be used for values of ‘x’ that fall within the range of the data used to generate the regression line (interpolation).
  • Extrapolation, or using the regression line to predict ‘y’ for ‘x’ values outside the data range, can lead to inaccurate predictions as it assumes the relationship between the variables continues unchanged.

Interpreting the Slope and Intercept

  • The y-intercept ‘a’ of the regression line represents the estimated value of ‘y’ when ‘x’ is zero. It might not always have a logical interpretation, especially when zero is not a meaningful value for ‘x’.
  • The slope ‘b’ represents the estimated rate at which ‘y’ changes for each unit increase in ‘x’. A positive slope indicates a positive correlation between the two variables, and a negative slope indicates a negative correlation.

Assessing Goodness-of-Fit

  • The closeness of the fit of the regression line to the data points can be determined using the coefficient of determination (R squared).
  • It ranges from 0 to 1, where 1 represents a perfect fit.
  • However, a high R-squared value does not necessarily mean that the regression model is an effective predictor - it might be overfitting the data.

The Assumption of Homoscedasticity

  • The assumption of homoscedasticity implies that the spread of residuals is roughly the same across all levels of the independent variable.
  • If this isn’t the case (if the spread of residuals increases or decreases with ‘x’), it signals heteroscedasticity. This can lead to inaccurate estimates and predictions.


  • The regression line is based on an assumption of linearity between the variables; if their relationship is not linear, a different type of regression analysis may be needed.
  • Outliers can significantly influence the slope and intercept of the regression line, hence the analysis should be accompanied by a careful check for outliers.
  • Even if the regression line fits the data well, it doesn’t imply a cause-effect relationship between the variables. It only shows a correlation.