Image (c) Nicole Gustavsen, CC BY.
Primary sources, broadly, are the sources that can bring us the closest to events that have happened in the world. In the sciences, these "events" are usually experiments, observations, discoveries, new modes of analysis, and new research methods. We learn about these events primarily via the publications of the people directly involved. Below are the most common types of primary research publication you are likely to encounter.
Research articles are extensive and detailed descriptions of scientific experiments, observations, and analysis carried out by the authors. These articles are published in peer-reviewed journals, or occasionally as chapters in edited scholarly books. They are the primary way that scientists learn about advances in their fields. If an article is very influential in its field, or at least very interesting, it is likely to be cited in the publications of other scientists working in the same areas, and perhaps analyzed, critiqued, or commented on in a secondary work of literature.
Conference proceedings are often “works in progress” originally meant to accompany the author’s lecture or poster at a conference. They may not have been peer reviewed in any meaningful capacity. The author may have later written a full article based on the conference proceeding.
Preprints are early versions of articles which have not been through a peer review process yet, but which the authors want their colleagues in the field to have access to anyways. These can be great sources of cutting edge information, but it’s important to remember that they still haven’t been reviewed fully in the peer review process.
Dissertations are written by doctoral students as the culminating evidence of their studies in graduate school. They are meant to be an original contribution of research to the author’s field. Dissertations are reviewed carefully by a committee of university faculty before a degree is awarded. While a full dissertation is often book-length, many authors will opt to publish parts of it as research articles.
A patent is a legal document providing evidence of intellectual copyright over an invention (usually a product, process, method, or composition), allowing the patent holder to exclude others from making, using, or selling the invention for a period of time. Patents include original evidence describing the invention, and are thus often considered primary. Once published by the US Patent and Trademark Office, they are freely available, although often difficult to locate.
Individual organizations produce a great quantity of original material documenting their operations that is never formally published. This is commonly referred to as “grey literature.” Grey literature that could be considered primary might include internal reports, technical documents, memos, and personal communications.
This section includes original data collected in the course of research projects. “Raw” implies the data hasn’t yet been cleaned up or manipulated. This includes numerical data, tables and charts, code, maps, transcripts, photos and drawings, lab and field notebooks, sound recordings, and even material samples. Raw data is sometimes shared by researchers who value open science, but this isn’t yet a norm so tracking it down can be challenging.
Image (c) Nicole Gustavsen, CC-BY.
Secondary sources are a step away from the "events" that primary sources document. Generally, these sources are commenting on, analyzing, interpreting, or evaluating primary sources. In college, many of the papers and articles that students produce are considered secondary sources. These types of sources help researchers contextualize what's happening in their field, and they can contribute to the direction of primary research by identifying longer-term trends and implications.
Reviews are a genre of article or book chapter which present an overview of the current state of research on a particular topic. The authors identify and analyze the most important discoveries, trends, and publications on that topic. There are different types of reviews that are prevalent in particular fields. Additionally, there are review journals that exclusively publish peer-reviewed review articles, and many edited scholarly books are collections of review articles.
Systematic reviews, which are most common to medical and health sciences, are a specific type of review article that attempts to answer a research question by systematically aggregating and reviewing the data from large numbers of existing primary research articles on a topic. Meta-analysis is a common statistical analysis used in systematic reviews.
Annotated bibliographies are a highly stylized form of literature review. Rather than being written as a single narrative, they present a list of sources (a bibliography) on a topic, and provide review and analysis (annotation) for each source as it relates to the theme of the bibliography. Other types of literature reviews are more likely to put selected sources into conversation with each other, by comparing and contrasting them together in an essay format rather than considering each individually.
Most, but not all, nonfiction scholarly books that are written entirely by one or two people (as opposed to edited volumes where each chapter is by a separate author) are works of secondary literature whose purpose is to provide commentary, analysis, and critique on a theme or topic. The authors are not reporting on new information they have discovered, but they are adding to the field with their intellectual examination of existing information.
Image (c) Nicole Gustavsen, CC-BY.
Information that is confirmed through the scientific process and through the vigorous debate played out in the literature eventually comes to be considered consensus knowledge. This type of knowledge is published in tertiary, or reference sources, whose main purpose is to present established information on a topic in easily digestible form, where it can be quickly referred to as people are working.
The purpose of an encyclopedia is to provide readers with a brief overview of established knowledge in a field. There is minimal analysis and no new information is being reported. Encyclopedias can vary widely in scope, from massive encyclopedias of everything, such as Wikipedia, to niche encyclopedias covering specific fields, such as the Encyclopedia of Chemical Technology.
Dictionaries exist to provide consensus definitions of words and phrases. There are many language dictionaries, which attempt to define every word in a language, and are usually pretty large. There are also smaller subject dictionaries that attempt to provide definitions of field-specific terminology, such as the Dictionary of Materials Science.
Textbooks are not usually written either to present wholly new information to the world, or to provide analysis or critical interrogation to existing information. The purpose of textbooks is to inform and educate students on the current state of knowledge in an area.
Handbooks and other tools that provide chemical property data are invaluable reference sources for chemists and other scientists who need chemical information. The data is originally discovered and published in the primary literature, but once established it is collated and presented in these handbooks and indexes where it can quickly be found and referred to as chemists are working.
These are not "sources" in the same sense as the other sources on this list, but they are all tools that allow us to locate primary and secondary literature. In that sense, many consider databases et al to be honorary tertiary sources.