Regression diagnostics Residual analysis: validating model assumptions Calculate the residuals and check the following Are the errors normally distributed? Normal probability plot Is the error variance constant? Plot of residuals against y Are the errors uncorrelated(time series data)? Plot of residuals against time periods Are there observations that are inaccurately recorded or do not belong to the target population? Double check the accuracy of outliers and influential observations Slide19Slide 19 Regression Diagnostics • Residual analysis: validating model assumptions • Calculate the residuals and check the following. – Are the errors normally distributed? • Normal probability plot – Is the error variance constant? • Plot of residuals against – Are the errors uncorrelated (time series data)? • Plot of residuals against time periods – Are there observations that are inaccurately recorded or do not belong to the target population? • Double check the accuracy of outliers and influential observations. y ˆ