Tuesday 24 September 2019

Date-Range Analyser

An important addition was made, some months ago, to the Measurement Tools page on this blog. That page contains an array of tools for converting between different units and for manipulating dates in a manner useful to historical research, and it is listed in several places on Cyndi's List.

One of the original tools is a birth-date calculator. It takes someone's age and a date of recording, and calculates a range of corresponding birth dates. For instance, using information from a census return, a death certificate, a marriage certificate, or even a tombstone. Once your  data has been entered then the 'Calculate' button will display the range of possible birth dates.

The age may be provided in years and/or months, but note that a non-zero months value implies greater accuracy. For instance, 12 months is considered more accurate than 1 year. The age is normally at "last birthday", but may be "next birthday", say for the early Canadian census years. The year can also be rounded down to the previous multiple of 5, say for the 1841 census of England and Wales.

The additional tool is a date-range analyser, and this brief post illustrates in what way it might be used. It is designed to compare multiple date ranges, and to look for the minimum overlap (i.e. the intersection between the ranges), or to show you the distribution of overlaps in case there are multiple peaks. The 'Analyse' button determines the intersections, and the 'Bar Chart' button then shows the distribution of overlaps graphically.

It can be used stand-alone, but it can also be used in conjunction with the birth-date calculator. The 'Remember' button on that tool will add the last calculated birth-date range to the list of ranges held by the date-range analyser.

In order to demonstrate this visually, let's introduce some example data: age information from four different documents.

Date of recording
Recorded age
Census of England and Wales
3 Apr 1881
Census of England and Wales
5 Apr 1891
Marriage certificate
1 Jul 1892
Death certificate
3 Jun 1945

Each of these can be entered into the birth-date calculator, and the 'Calculate' button pressed, followed by the 'Remember' button. All four of the potential ranges will then have been passed over to the date-range analyser.

If you press 'Analyse' there then it will determine the minimum overlap(s). If you then press 'Bar Chart' then it show the distribution of overlaps graphically, as below.

You can see from this that there are two distinct peaks. This information was once used to help prove that the data related to two distinct individuals who just happen to have the same name and lived in a similar locality.