When the boundaries of council wards change, it is important to have an idea of how many councillors each party would have had if the new boundaries been in place last time the seats were contested.
This way, the change in the number of councillors elected for each party can be calculated on a like-for-like basis.
This is not an exact science. When boundaries are redrawn and an area is moved from one council ward into another there is no official record of how that area voted.
This is where the experience and expertise of analysts comes in.
To calculate the notional results, analysts use maps of the old and new boundaries. They look at the area, and use local knowledge and professional judgement to help them work out how people are likely to have voted.
Using this information, they recalculate the results of the last election to show how many councillors each party would have had if the new boundaries had been in place.
It is this figure that is used by the BBC to calculate change in these areas.
You can see if an area has had boundary changes in the ‘About this council’ section at the bottom of each council results page.
