In genealogy, each record we use to build our case is referred to as a source, and these sources fall into one or more different categories. It is important to know which types of sources we have to understand the value of each. Some records are more or less reliable than others, and carry more or less weight of evidence for events than other records.
Original sources are the first record of the event, and are more likely to have correct information. Derivative sources are those compiled or derived from other sources, or multiple sources. Each time information is copied or transcribed or retold the likelihood for errors increases, and so derivative sources are considered less reliable than original sources. In some cases the original source may no longer exist and all that remains is the derivative source.
A primary source was created during the time of the event, likely by people who had firsthand knowledge of the event, and not based on other sources. It is more valuable to us as researchers (though not guaranteed without errors). A secondary source is one that was created later after the fact, by someone who was not a witness to the event or did not experience it during their lifetime (and therefore may be less reliable), and may be based on other sources. Secondary sources tend to explain, analyze, or summarize primary sources. A marriage certificate would be a primary source as it was created at the time of the event, but a book about marriages in the area that included a record of the couple would be considered a secondary (and derivative) source. While primary sources should be given more weight, proper research should use all available sources that are proven to be authentic and form a logical case for the narratives you are forming.
Furthermore, a record can be both primary for one event, and secondary for another event. For example, while that marriage certificate can be a primary record for the marriage date of the bride and groom, it can be a secondary source for the groom’s date of birth. We are not positive the person creating the marriage certificate or the informant was a firsthand witness to the groom’s birth, so we must assume it is secondary information.
Also note that any time a primary source is transcribed or copied, that copy is considered a secondary source (though some genealogists refer to these transcriptions or abstracts as a derived primary source). An original document, such as a census record, is still considered primary and original when viewed online at a database like Ancestry.com. However, if you were looking at Ancestry.com’s abstract of the record, which is a transcription of the information found in the census, that would be considered secondary by most genealogists today. It is also considered an original source when it is an exact copy of an original, like a photocopy or a printout.
Direct Evidence is explicit or specifically stated, while indirect evidence is implied or not explicit, but allows you to draw a conclusion. Like indirect evidence, negative evidence is not explicit. When the absence of information in an area where it was expected can lead to a conclusion. If a wife was shown in the 1930 US Census but does not appear in the 1940 US Census, this can be both negative and indirect evidence of her death, but still must be proven as there are other reasons (divorce, travel, etc) she may be missing. This should be taken as a clue that would lead us to search for a death certificate from this time frame.
A photograph taken by your grandmother would be considered an original source, and also primary. It could also be considered a home source, as it was likely found in your residence or that of a relatives.
While we should always try to find the highest valued types of sources, we know it’s not always possible, and we accept lesser valued types of sources (as long as we evaluate the evidence and our research is exhaustive, as per the Genealogical Proof Standard).
Also if there is a conflict of information within our research we are trying to resolve, we should give more weight to primary, original sources over derivative and secondary sources. For example, if the dates are conflicting you should believe the birth date on a birth certificate over the birth date found in a family history book created many years later by a distant cousin, though keep notes of the conflict.
You should also give more weight of evidence to sources created closest to the event. A transcription of a gravestone found in a town history book and recorded a few decades after the burial should be “heavier” than a transcription made in the cemetery recently.
Examples of Primary Sources
Things people say: interviews, audio recordings, video recordings
Things people write: journals, diaries, postcards, letters, autobiographies
Photographs
Original vital documents: birth certificates, marriage certificates, death certificates
Family Bible with handwritten notes made at the time of events
Newspaper articles from the time of the event
Court records and legal documents
Church records
Census records
Passenger lists
Examples of Secondary Sources
Books with collections of records
Town and city histories (created later, not by those who experienced it)
Biographies
Journal articles
Other people’s research
Transcriptions and translations
See also: Citing Sources and Home Sources and The GPS