Verification and validation of computer simulation models is conducted during the development of a simulation model with the ultimate goal of producing an accurate and credible model. "Simulation models are increasingly being used to solve problems and to aid in decision-making. The developers and users of these models, the decision makers using information obtained from the results of these models, and the individuals affected by decisions based on such models are all rightly concerned with whether a model and its results are "correct". This concern is addressed through verification and validation of the simulation model.
Simulation models are approximate imitations of real-world systems and they never exactly imitate the real-world system. Due to that, a model should be verified and validated to the degree needed for the model's intended purpose or application.
The verification and validation of a simulation model starts after functional specifications have been documented and initial model development has been completed. Verification and validation is an iterative process that takes place throughout the development of a model.
Verification
In the context of computer simulation, verification of a model is the process of confirming that it is correctly implemented with respect to the conceptual model (it matches specifications and assumptions deemed acceptable for the given purpose of application). During verification the model is tested to find and fix errors in the implementation of the model. Various processes and techniques are used to assure the model matches specifications and assumptions with respect to the model concept. The objective of model verification is to ensure that the implementation of the model is correct.
There are many techniques that can be utilized to verify a model. These include, but are not limited to, having the model checked by an expert, making logic flow diagrams that include each logically possible action, examining the model output for reasonableness under a variety of settings of the input parameters, and using an interactive debugger. Many software engineering techniques used for software verification are applicable to simulation model verification.
Validation
Validation checks the accuracy of the model's representation of the real system. Model validation is defined to mean "substantiation that a computerized model within its domain of applicability possesses a satisfactory range of accuracy consistent with the intended application of the model". A model should be built for a specific purpose or set of objectives and its validity determined for that purpose.
There are many approaches that can be used to validate a computer model. The approaches range from subjective reviews to objective statistical tests. One approach that is commonly used is to have the model builders determine validity of the model through a series of tests.
Naylor and Finger [1967] formulated a three-step approach to model validation that has been widely followed:
Step 1. Build a model that has high face validity.
Step 2. Validate model assumptions.
Step 3. Compare the model input-output transformations to corresponding input-output transformations for the real system.
Face validity
A model that has face validity appears to be a reasonable imitation of a real-world system to people who are knowledgeable of the real world system. Face validity is tested by having users and people knowledgeable with the system examine model output for reasonableness and in the process identify deficiencies. An added advantage of having the users involved in validation is that the model's credibility to the users and the user's confidence in the model increases. Sensitivity to model inputs can also be used to judge face validity. For example, if a simulation of a fast food restaurant drive through was run twice with customer arrival rates of 20 per hour and 40 per hour then model outputs such as average wait time or maximum number of customers waiting would be expected to increase with the arrival rate.
Validation of model assumptions
Assumptions made about a model generally fall into two categories: structural assumptions about how system works and data assumptions. Also we can consider the simplification assumptions that are those that we use to simplify the reality.