# Key definitions

# Key definitions#

Modeling Hub

a consortium of research groups working together on a common set of modeling tasks, with the goal of developing ensemble models to provide stakeholders with a single model output representing uncertainty across different modeling assumptions and frameworks.

Round

a time-period for which a set of specific model outputs are solicited. Rounds define the “cadence” of submission for a modeling hub. For example, some hubs might accept daily submissions, where each day is considered a different round. Other hubs might have one round every month, with a submission period that may be open for multiple days.

Target

a quantitative outcome of interest for a modeling Hub. For example, “incident case counts”. Targets typically (and sometimes implicitly) refer to a value of an observable variable in a given window of time, a given location, and possibly other stratifications (such as age group).

Task

a definition of the goals of a modeling effort, possibly including conditions, assumptions, and targets. Some tasks may be fixed across rounds, as for forecast hubs that regularly solicit predictions for a set time horizon in the near-term future. Other tasks may be more variable, as for scenario hubs that model hypothetical futures with differnt assumptions in different modeling rounds.

Team

a group of individuals developing modeling software to generate models in response to tasks coordinated by modeling hubs.

Mathematical Model formulation/structure

a statistical or mathematical formulation of a model.

Modeling software

code that implements the mathematical model formulation/structure used by a team to generate model outputs. Each team may have multiple instances of software.

Model output

a set of target results generated in response to some modeling task for a specific round. A model might be the result of a single team’s response to the task, or it might be an ensemble of results representing outcomes of multiple efforts

Nowcast

model output that provides estimates/predictions of partially observed or unobserved values at the current date from a data stream prior to the current date. Nowcasts should be set up so that they can be evaluated for accuracy based on comparisons with the eventually observed complete data.

Forecast

a specific quantified prediction of an observable event or trend that has not yet been observed, conditional on data that has been observed up to a specified time. Forecasts should be set up so that they can be evaluated for accuracy based on comparisons with the eventual observed data.

Scenario

a description of a possible future to be modeled, described in terms of models parameters that might be varied, such as transmissibility, vaccine adoption, vaccine efficacy, the emergence of a new variant, etc.

Scenario projection

model output that provides estimates of future observations of future trends conditional on specific assumptions about a given scenario. Scenario projects are challenging to evaluate against future observed data since the assumptions under which scenarios were generated likely will never have been exactly met.