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A one-name study is a project researching a specific surname, as opposed to a particular pedigree (ancestors of one person) or descendancy (descendants of one person or couple). Some people who research a specific surname may restrict their research geographically and chronologically, perhaps to one country and time period, while others may collect all occurrences world-wide for all time.
A one-name study is not limited to persons who are related biologically. Studies may have a number of family trees which have no link with each other.
Findings from a one-name study are useful to genealogists. Onomasticians, who study the etymology, meaning and geographic origin of names, also draw on the macro perspective provided by a one-name study.
Many people conducting family history, genealogical or onomastic research may conduct a one-name study of a surname in a given period or locality quite informally.
A full one-name study can be daunting, particularly if the surname is very common. Conversely, a rare surname can be difficult to trace. Since such studies are usually conducted by individuals as a pastime, they are generally feasible only when a surname is not used by more than a couple of thousand contemporary people, so that the total historical data-set is numbered in the low tens of thousands. Where a surname is used by hundreds of thousands, or millions of people, it would be practically impossible to differentiate these persons using national-index data alone.
In some countries, one-name studies are impossible, since surnames are not used at all or in the case of names such as Singh may represent religious practice rather than an ancestry. Since a majority of human societies are patronymic, one-name studies generally focus on male succession and ignore family relationships through marriage.
Some researchers are satisfied to collect all information and group it geographically, approximately representing the different family groups. Others attempt to reconstruct lineages. Because of the wider scope of a one-name study, and transcription or OCR errors in the indexes employed, lineage-making cannot be done with as much accuracy as in a single-family genealogy.
In most one-name studies, a united lineage will not be discovered, but a broad perspective can be achieved, giving clues to name origin and migrations. Many researchers are motivated to go beyond the one-name-study stage and to compile fully researched, single-family histories of some of the families they discover.
In most other countries, one-name studies are much more difficult. Where civil-registration indexes are open to public search, they may not be online or gathered in the national capital, but are scattered through the states, as in Australia, or towns, as in France and the United States. In many countries, such as Germany, civil-registration and census data are regarded as a state prerogative: vital data are only available to the persons concerned and 19th-century census returns are not available at all.
One-name studies of the United States have become feasible thanks to the recent availability of online indexes to 19th-century and early-20th-century censuses.
More limited one-name studies can be conducted using other national indexes including:
- telephone and address directories
- registers of wills or deceased estates
- electoral rolls
- land possession records
- military service indexes
To obtain surname data from the 18th century and earlier, one-name researchers employ the International Genealogical Index (IGI) and vital records indexes compiled by The Church of Jesus Christ of Latter-day Saints as well as catalogs to national archives.
One-name studies are generally rounded out with a miscellany of information drawn from national bibliographies, archival catalogs, patent databases, reports of law cases, tax lists, newspaper indexes and web searches. A one-name researcher is also expected to report on the linguistic origins of the surname and its use in placenames and corporate names.
UK surnames 
Civil registration indexes of births, marriages and deaths in England and Wales (for the period from 1837) and Scotland (from 1855) are in the public domain, and anyone has been able to apply to see the details of any birth, marriage or death. For the period before civil registration, in principle back to 1538 in England and Wales and 1533 in Scotland, parish registers have recorded birth and/or baptisms, marriages and deaths and/or burials. These are also been freely available, although the survival of such registers is less likely as we reach back to the earliest dates of this period. It has been possible to extract a complete data-set of a given surname from these public records. Simple profiles of most 20th-century persons with the surname in England and Wales can thus be drawn up without needing any contact to the persons concerned.
Censuses have taken place in England, Wales and Scotland since the 1800s. Information from the 10-yearly censuses from 1841 until 1911 is available and facilitates the linking of surname data into family groups.
The civil registration index books were scanned and made available online in 2004 by 1837online, now findmypast.com, and a partial index has been transcribed by volunteers for the FreeBMD website. Records for Scotland can be searched at the ScotlandsPeople website, and this means that a one-name study with a British focus can be conducted from anywhere in the world.
While most one-name studies are conducted as a pastime, rather than as an economic activity, the sheer volume of information to be organized may require semi-professional data-processing and publishing skills.
The data must be carefully structured. An accurate copy of the original indexes must be drawn up, and updated when they are amended. Errors and conflicts in the indexes are noted. Links to those tables appear in the roll of individual persons.
To avoid retyping large volumes of data by hand, one-name researchers are often skilled at data scraping and automated reformatting.
Many one-name researchers keep data tables in computer spreadsheets because it is possible to see hundreds of items on a single screen and use thinking power to detect patterns. Family Tree software is used by many researchers to collate and define family trees. Others employ relational database software.
Motivation and support 
One-name researchers often begin a study in the hope that obtaining a massive data set will give them sufficient perspective to break through a barrier in their own family history research. Some are motivated by the belief, only rarely borne out, that kinship can be documented among all persons sharing a surname. Like most other collecting pastimes, a one-name study often becomes compulsive, without regard for the original motivation.
The principal organisation advising on such research is the Guild of One-Name Studies which was established in Britain in September 1979. The Guild now has over 2,000 worldwide members conducting studies of individual surnames and their variants and has regional organizers in several nations.
Traditionally, publication of definitive research is undertaken by printing a book or by publishing a one-name periodical. Such publications are often sponsored by formally established one-name groups. The Federation of Family History Societies includes several One-Name Societies, whilst the Guild of One-Name Studies has many members who are associated with such organisations. Advice on setting up a one-name group appears in a short booklet, "One-Name Family History Groups" by Derek Palgrave published by the Halsted Trust in 2008.
Today many studies are presented online, since the data can be continually updated and made available worldwide.