This is a more rigorous test than just validating that the screens display information correctly. In addition to the basic functional testing, other important types of testing need to be considered, such as stress and performance, reliability, security, and recovery. All of these system attributes can be adversely affected by a system change. To accurately test these attributes, automated tools are often needed to simulate the volumes of transaction involved in a given time period. This is a challenge because few companies have adequate testing environments in place.
Most test environments are subsets that can handle only a fraction of the production environment.
When you consider testing the interaction between systems, it becomes apparent that either a larger test environment will be needed or a smaller scale test will be performed. There are risks on both sides of this issue. On one hand, you can spend large amounts of time and money to create a dedicated test environment which will take resources from other areas in the organization. On the other hand, inadequate testing puts the organization at risk from the effect of software defects found in production. The only way to determine the degree of risk is to perform a risk assessment.