4.1.5 Capturing Business Rules
One way of enabling our thinking about dynamic hypotheses is to state formally what we understand to be the mechanisms causing changes to occur over time. Here we must write down carefully and deliberately our hypotheses. It is essential to use clear and unambiguous language. An essential pre-cursor step to developing dynamic hypotheses is to define formally what we believe to be the business rules or rules governing behaviour of the problem. Examples are:
- Initial Population: ‘The initial size of the population is the size of the population at the start of the period under consideration. We identify this point in time as the start of our simulation.’
- Mechanisms Affecting Population: ‘The population can only change through the following mechanisms:
- Children are born to fertile females of child-bearing age.
- People die as a result of old age or natural causes for which patterns are well known: the effects of pandemics or catastrophic events are not included.
- People choosing Charmville as a place to live can migrate, but the rate of arrival is dictated by the availability of housing which is a consequence of local government policy.
- People choosing to leave Charmville can do so freely. They can leave at any time they choose. The numbers of people choosing to leave appears to be independent of population size.’
- Birthing: ‘About half of the population are female and a fraction of females are of child-bearing age. At any time children are born at a rate dependent upon this fraction and the size of the population.’
- Dying: ‘People die at fundamentally constant rates: the total number dying each year is, therefore, determined by the size of the population.’
- Migrating-in: ‘At any time there is always a surplus of people working at Metropolis who would want to live in Charmville to enjoy its lower cost of living and idyllic lifestyle: migrating-in is effectively controlled by the availability of housing and a program of land release both of which are controlled by Charmville’s local government agencies.’
- Migrating-out: ‘The rate of leaving Charmville is historically constant, appearing not to be linked to population. Rather, this may be linked to annual employment turnover at Metropolis.’
These are expanded into the form of more complete dynamic hypotheses below. Each takes the form of a storyline and description of the contribution of each to the structure of the problem.