The information displayed in the summary table is the result of three subsequent steps.
- Firstly, all records in the database including a population data point are used to fill in as many elements of the table as possible. On one hand, if only one active record exists for a given combination of city and time period, then its value is quite simply displayed in the relevant cell. On the other hand, if several records can be found, e.g. for the same city at the same point in time but from different sources, or for the same city from the same source but at different points falling within the same time period, then their values are averaged and the result is displayed in the relevant cell.
- Secondly, all records in the database including information on the foundation year for each city are also used to set to zero the value of all cells prior to the one that includes the foundation year. These two steps result in a partly populated matrix incorporating all hard data actually available, but still including a number of gaps.
- Finally, the gaps are be filled by interpolating between the values available on the two sides of each gap. All interpolations are conducted on a linear basis.