John M. Drake & Pej Rohani
A forecast is a quantitative statement about an event, outcome, or trend that has not yet been observed, conditional on prior data that has been observed.
Adapted from: Lauer, S.A. et al. 2020. Infectious disease forecasting for public health. In Population Biology of Vector-borne Diseases edited by J.M. Drake, M.B. Bonsall, and M.R. Strand. Oxford University Press.
Typically, it is assumed that forecasts are of future events, but this definition allows for “retrospective” forecasts, including nowcasts.
Probabilistic forecasts are an important special case.
Both mechanistic and statistical models may be used for forecasting.
Statistical models typically outperform mechanistic models.
Discussion question: What do you expect to be the strengths and weaknesses of these two kinds of models?
A 2014 scoping review of influenza forecasting models found that 17/35 models used compartmental models and 18/35 studies used statistical models
Chretien J-P, George D, Shaman J, Chitale RA, McKenzie FE 2014. Influenza Forecasting in Human Populations: A Scoping Review. PLoS ONE 9(4):e94130. doi:10.1371/journal.pone.0094130
Targets are the unknown (but verifiable) quantities that forecasts make quantitative statements about (i.e. an event, outcome, or trend).