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Citing sources is the hallmark of professional and scholarly communication. As a scientist or engineer, you communicate how you built your work and reached your conclusions. By citing sources, you:

• Lend validity to your own research approach
• Link your conclusion/contribution/idea to its context
• Give the original creator credit
• Permit your reader to verify your claims and pursue more information

Citing sources encourages you to think. By documenting how others' ideas connect to yours, you get the concepts more firmly in your head. Anything else is cheating yourself on your education.

Plagiarism is bad. Whether you content-scrape, buy another's paper, or just don't keep track of what you're doing, it's a breach of professional ethics if intentional, and also a sign of incompetence if unintentional. Either way, be aware of what constitutes plagiarism, and don't do it.

Each research field has its practice on when to cite a source. You can develop a better understanding of practices in a specific field by reading more. Here are five principles with examples to get you started*.

(All examples used below are from a highly cited article by Staiger, P. et al. ** ) You must cite sources if you:
 

1. Use facts or data to support your own argument. The only exception is the generally known and accepted constants or common knowledge, e.g. room temperature is 298K.
   
   E.g. “Magnesium is an exceptionally lightweight metal. With a density of 1.74 g/cm³, magnesium is 1.6 and 4.5 times less dense than aluminum and steel, respectively [16].” … …
   “[16] DeGarmo PE. Materials and processes in manufacturing, 5th ed. New York: Collin Macmillan; 1979.”
    
2. Summarize others’ ideas, thoughts, research stories or conclusions in your own words.
   
   E.g. “McBride reports on the use of screws, pegs, plates and bands prepared from magnesium–aluminum–manganese alloys to secure 20 fractures and bone grafts [35].”… …
   “[35] McBride ED. Absorbable metal in bone surgery. J Am Med Assoc 1938;111:2464–7.”
    
3. Paraphrase others’ sentences using your own words. (Note: simply changing the order of the words and/or tense is considered plagiarism not paraphrasing.)
   
   E.g. “Significantly increased (P<0.05) bone area was observed in all groups with magnesium-based implants at weeks 6 and 18, in comparison to the polymer control [31].”… …
   “[31] Witte F, Kaese V, Haferkamp H, Switzer E, Meyer-Lindenberg A, Wirth CJ, et al. In vivo corrosion of four magnesium alloys and the associated bone response. Biomaterials 2005;26:3557–63.”
   Original sentence in the cited article - “There were significant increase (p<0.05) in the mineralized bone area in all groups with magnesium implants at 6 and 18 weeks postoperatively compared to the control group (SR-PLA96) (Fig. 6).”
    
4. Would like to offer supplemental information to your readers without repeating it in your writing.
   
   E.g. “Moreover, magnesium is essential to human metabolism and is naturally found in bone tissue [21], [22], [23], [24], [25] and [26].”
   The reference 21 to 26 cited are not excerpted here. The sentence above did not explain how magnesium is essential to human metabolism but implied that the readers can find more about it in reference 21 to 26.
    
5. Quote others’ sentence, figure or table directly. In science and engineering, direct quotation is rarely used for research articles unless the original wording of a sentence has significant impact on the scientific meaning. Tables and figures are more likely to be quoted in review articles with the permission of the copyright holders, usually the journal publishers.
   
   E.g. “Fig. 2. SEM micrograph of a magnesium material with porous microstructure produced using space-holding particles. Reproduced from Wen et al. [62].”… …
   “[62] Wen CE, Yamada Y, Shimojima K, Chino Y, Hosokawa H, Mabuchi M. Compressibility of porous magnesium foam: depen-dency on porosity and pore size. Mater Lett 2004;58:357–60.”

Besides the above examples, a general principle is when in doubt, cite your sources.

*Princeton University. Academic Integrity at Princeton: When to Cite Sources. https://www.princeton.edu/pr/pub/integrity/pages/cite/ (accessed September 9, 2016).

**Staiger, M. P.; Pietak, A. M.; Huadmai, J.; Dias, G., Magnesium and its alloys as orthopedic biomaterials: A review. Biomaterials 2006, 27 (9), 1728-1734. http://dx.doi.org/10.1016/j.biomaterials.2005.10.003

Citation management software, a.k.a. reference management software or personal bibliographic management software,  can help you with:

• organizing references, PDFs, images, data sets, webpages etc. 
• taking notes on references and annotating PDFs and searching notes later
• formatting bibliographies and in-text citations in hundreds of citations styles
• sharing references and collaborating on projects with colleagues

This guide will overview a few pieces of citation management software, including Zotero, Mendeley, EndNote (Web and Standalone), and Papers,  to help you choose and use them. Remember, if you need to use more than one throughout your academic career, there are methods to transfer your data from one to another. 

Biology

Council of Science Editors (CSE) Style

Chemistry

American Chemical Society (ACS) Style

Geology

Suggestions to Authors (STA) style by United States Geological Survey (USGS)

Geology Society of America (GSA) style

Liberal Arts

Modern Language Association (MLA) Style

Chicago Manual of Style

American Psychology Association (APA) Style

Mathematics

American Mathematics Society (AMS) Style

Physics

American Institute of Physics (AIP) style

Computer Science

Institute of Electrical and Electronics Engineers (IEEE) Computer Society recommends Chicago Manual of Style

Economics

Chicago Manual of Style. See recommendations for Liberal Arts.

Engineering

American Society of Civil Engineers (ASCE) and Institute of Electrical and Electronics Engineers (IEEE) recommend Chicago Manual of Style

Government Information

Metallurgy and Materials Science

See recommended styles for Chemistry or Engineering

Mining

See recommended styles for Engineering

Petroleum

American Association of Petroleum Geologist (AAPG) style

Society of Petroleum Engineers (SPE) style