There are a number of metrics that can be used to estimate the quality of a design:

• Cohesion
  The extent to which the function of a component is focused on a single concept. Some possible points of focus are a system, an event, a stimulus, a response, access to the state of an object and so on. Cohesion is a property of the component.

  The goal is high cohesion. High cohesion means that each element in a component is essential for the component to obtains its purpose.

• Coupling
  The degree of interdependence between interacting components. Coupling is a property of the associations between components.

  The goal is low coupling. Low coupling means that the interface between two components is relatively simple and that ability of one component to use another component does not require detailed knowledge about the internal structure or operation of the component.

• Fan-out
  The number of components that act as servers to a single client. A high number of servers increases the probablity that the client may need to be modified.

  The goal is low fan-out.

• Fan-in
  The number of components that act as clients to a single server. A high number of clients increases the possible impact of a change to the server.

  The goal will depend on the cohesion of the server component. High fan-in is permissible with a highly cohesive server.