The skill required to make reliable judgments from such indexes without consulting the original archival documents is developed from long family history experience.
It is usual to include all spelling variants of the surname, unless there is clear evidence that a close spelling has a distinct geographic or family origin. A one-name study is not limited to persons who are related biologically, but also embraces those who acquire the surname by marriage or adoption or through slavery.
Findings from a one-name study are useful to genealogists, who burrow deeper than indexes, consulting the historical sources so as to write pedigrees or descendancy charts of single families that are usually subsets of the surname group. Onomasticians, who study the etymology, meaning and geographic origin of names, also draw on the macro perspective provided by a one-name study.
Because the index of births has, since 1911, included the mother's maiden surname, and the marriage index since 1912 has displayed the surnames of both partners, it is possible, with a complete data-set, to match every man's marriage to the occasions when he had a child. This information could be acquired at no cost in London. 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.
Using this groundwork, it is possible to hypothesize crude lineages and extend these back through the 19th century by collecting every instance of the chosen surname from indexes to the 10-yearly censuses that began in 1841.
The data can be refined using 19th century civil-registration data. All persons living in England and Wales were obliged from 1837 onwards to register births, marriages and deaths. Quarterly index books listing the surnames, first names and districts where these events took place were thereafter kept for public use in London. Since compliance was mixed at first, and the data-fields in the 19th century indexes are more limited than for the 20th century, those index books alone are not sufficient for reconstituting families.
The 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.
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:
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.
A full one-name study can be daunting. Since such studies are usually conducted by individuals as a pastime, they are generally only feasible 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.
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.
Family-tree software is not suitable for one-name studies, though it may be useful later if genealogical research is to be conducted and lineages confirmed. 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. Others employ relational database software.
The principal organization 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.
As an initial step, a searchable version of the database can be offered online. Traditionally, publication of a full report was done by printing a book or by bringing it out in parts in a one-name periodical. Today many studies are presented online, since the data can be continually updated, is available worldwide and is likely to be preserved by Archive.org and other services long after the website expires.
There is little value in merely publishing index extracts without annotation or links: this may even breach the index compiler's copyright. Instead, the study can publish rolls of persons, fully referenced and arranged in geographical categories, either alphabetically or in the order of such fragmentary lineages as can be discovered.
At the same time, most one-name researchers believe they are honour-bound to return some good to society. A one-name study not only benefits from the availability of public records, often at no charge, but also from the generous advice and assistance of hundreds of strangers who are consulted for information in the course of the study. Publishing the historical part of the research and promptly answering inquiries is the best way to repay this use of the commons.