Historical demography is a branch of population studies that can be defined as "the application of demographic techniques to historical data," but there is no clear definition of the era when this data became "historical" data, nor is there a universally agreed date at which current demography replaced historical demography. Typically, these sets of records are more than a century old. Another factor that distinguishes historical demography from other branches of demographic studies is a set of techniques, such as family reconstruction. Family reconstruction is a technique whereby historical records, usually parish registers, are used to reconstruct the lives of entire nuclear families, in order to examine demographic concepts such as fertility, marriage, and mortality.