EVI: Stata module to compute Epidemic Volatility Index (EVI) for detecting epidemic waves

evi is based on the volatility of the newly reported cases per unit of time (ideally per day) and issues an early warning when the rate of the volatility change exceeds a threshold ('c'). Issuance of consecutive early warnings is a strong indication of an upcoming epidemic wave. EVI is calculated for a rolling window of time series epidemic data ('lag'). At each step, the observations within the window are obtained by shifting the window forward over the time series data one observation at a time. The user should provide the minimum rise in mean cases between two consecutive weeks ('r') that, if present, should be detected.