It is important to remember that the map can only give an indication of the number of people in a parish
in any given year. There is insufficient data available to provide an exact figure. The data here is derived
from recorded baptisms taken from parish registers. A typical family in a parish might baptise children
every two years over a period of perhaps 10 years. There will then be a gap of 10-20 years with no baptisms
until the first generation of children have grown up and started families of their own. With a pattern like this,
simply displaying the baptism numbers for a single year would give a very disjointed picture of the family. We
therefore display for any year, the number of baptisms in that year, together with a linearly weighted average
of the numbers in the 9 years before and after the displayed year. Visually this gives a more accurate impression
of the family presence.
The scaling used to indicate the presence of a family represents the mean number of baptisms per year in the parish.
This is depicted on the map by the colour shown for the parish, which follows a smooth
progression from white, through yellow, orange, red and brown, to black, the latter corresponding to a mean rate
of one baptism per year. The exception to this smooth progression is the transition from a zero presence (green) to the first
appearance of a baptism for the chosen surname (white), where the strong colour change makes it easy to see the
point at which the first family member appears in the parish.
At first sight it might be assumed that one baptism per year represents a rather
small presence of the family in a parish but this is not the case. By 1800, life expectancy at birth was around 40, so a sustained rate of one baptism per year
typically corresponds to around 40 individuals present in the parish at any one time and represents more than five individual households.
Note that because the data for this map is derived from the OFHS Search Service baptism index, no data will
be shown for the small number of parishes that are not yet included in this index (approximately 3% at the time of writing).
You can see which these
are from the Parish Register Coverage Map on
the main Search Service section of this web site. It is usually possible to infer a presence in these few
parishes from the levels in adjoining parishes.