# Conceptual basis¶

EasyVVUQ was created as part of the VECMA project. The aim of this project was to make state of the art VVUQ algorithms available for use in HPC applications (and specifically multiscale models). The basis of making generic tools within VECMA is the idea of Patterns, which are:

“abstractions that describe, in a non-application and non-domain specific manner, a workflow or algorithm for conducting validation, verification, uncertainty quantification or sensitivity analysis”.

Making use of Patterns in practice requires that they are decomposed into components which can be flexibly combined to implement a range of algorithms.

EasyVVUQ is designed around a breakdown of such workflows into four distinct stages (see Figure 1); Sampling, Model Evaluation, result Aggregation, and Analysis. In an HPC context the model evaluation step is generally equivalent to the execution of a (computationally expensive) simulation. The actual simulation execution is beyond the remit of the package but EasyVVUQ is designed to wrap around simulation execution, providing functions to generate input (an Encoder) and to transform simulation output into common formats for analysis (a Decoder). Below we describe the components of EasyVVUQ designed to perform each step in more detail.

## Parameter description¶

The first step in our generalised workflow is a description of the model parameters and how they might vary in the sampling phase of the VVUQ pattern. Typically the user will specifying all numerical parameters, the distribution from which they should be drawn and physically acceptable limits on their value.

## Campaign¶

EasyVVUQ workflows are coordinated by an object called a Campaign. This contains a common database, the CampaignDB, which contains information on the application(s) being analysed alongside the runs mandated by the sampling algorithm(s) employed. It also stores the decoded results of the simulations themseleves. The Database is the central location where all information about your campaign is kept. The Campaign handles all validation and is transfers information between each stage of the workflow.

## Elements¶

Within VECMA software components that can be reused in a wide range of application scenarios are known as Elements. Within EasyVVUQ we provide five classes of Elements (Samplers, Decoders, Encoders, and those providing collation, for the aggregation step, and Analysis functionality) which we describe below.

## Samplers¶

A Sampler populates the CampaignDB with a set of run specifications based on the parameter description provided by the user. Each Sampler is designed to employs one of a range of algorithms, such as the Monte Carlo or Quasi Monte Carlo approaches (Sobol, 1998). They deal with generic information in the sense that all parameters use the nomenclature and units provided by the user rather than anything specific to any application or workflow.

## Encoders¶

The role of an Encoder is to convert generic parameter descriptions into inputs (for example configuration files) which can be used in a specific application. Included in the base application is a simple templating system in which values are substituted into a text input file. For many applications it is envisioned that specific encoders will be needed and the framework of EasyVVUQ means that any class derived from a generic Encoder base class is picked up and may be used. This enables EasyVVUQ to be easily extended for new applications by experienced users.

## Decoders¶

The role of a Decoder is twofold, to record simulation completion in the CampaignDB and to extract the output information from the simulation runs. Similarly to an Encoder, a Decoder is designed to be user extendable to facilitate analysis of a wide range of applications.

## Analysis¶

The final goal of any VVUQ workflow is an analysis which provided information on the simulation output across a range of runs. Different types of analysis (for example bootstrapping of multiple runs from varied initial conditions) are, or will be, provided by EasyVVUQ.