Visual representation of the difference between rolling and expanding window approaches to calculating early warning signals (EWSs - A vs C) in a hypothetical transitioning time series. Solid bars indicate the changing window. Panels B and D then indicate the quantity that represents a ‘warning’. For rolling windows (A, B), this warning is a strong Kendall’s Tau correlation of EWS indicator values with time. Whereas, for expanding windows (C, D) a warning occurs when the standardised EWS value exceeds a 2σ threshold.