Fundamentals of Statistics contains material of various lectures and courses of H. Lohninger on statistics, data analysis and here for more.

Error Bars

Error bars are a useful instrument to indicate the range of uncertainty of measurements. Error bars should be included to any chart whenever proper information is available. The error bars should indicate the confidence interval of a certain measurement. In the figure below you find an example of a graph showing error bars along with the data. Error bars can only be estimated by repeated measurement - thus, drawing correct error bars, always means (much) more effort.