It is important to document every idea, statistic, quote or paraphrase that you use from somewhere else, to give credit to authors, let your reader know the basis for what you're saying, avoid plagiarism. This is called citing or referencing your sources. In science and engineering, we cite each source in two places:
To cite a source, you can't just paste the URL into your paper - you need to include specific information in a specific order. A citation style is a particular way of formatting and presenting references and citations The information to include depends on what kind of source you are citing - a book, journal article, news website, etc. So it's important to know what kind of source you have!
There are different styles used in science and engineering, and your professor may require a specific style, but they always want you to follow one in particular. The two most common citation styles in science and engineering are APA style (an 'author-date' style) and IEEE (a numbered style). There are other styles in particular fields (ASME ASCE) that are similar to these. The key features are summarized below.
Author surname, A.B. & Author surname, J.K. &... (Year or date). Title of the item. Publication, conference, website or organisation, Details (dependent on type of item). URL or DOI.
For more help, refer to the examples further down this page, an Online guide with examples prepared by Kwantlen Polytechnic, or the Official APA website with more examples
Bibliography citations are ordered numerically from [1] to [n].Generally, citations follow the format below, however the the details vary by item type.
[#] A.B. Author surname, J.K. Author surname and... , "Title of the item". Publication, conference, website or organisation, Details, Year and possibly month. URL or DOI.
For more help, refer to the examples further down this page, Guide to IEEE style by Purdue University, or the Official IEEE guide (note that the official guide is not that easy to understand)
You can create these formatted citations
But no matter how, you need to make sure the citation is correct and has the necessary info. Tools sometimes make mistakes, especially with references that aren't journal articles and conference papers.
You are required to cite any source you use, whether it's a journal article, a textbook, a video, or or an image you found on the internet.
You aren't expected to provide a citation for facts, definitions or equations that are well-known to everyone in your field. For example, in a class on machine learning, you wouldn't normally need to cite a short definition of 'supervised learning'. But if you are writing several paragraphs on the steps in a supervised learning process, you would need to cite a source for that - even if it's your course textbook!
If in doubt, it's better to be cautious and cite more.
This page from McMaster University gives more guidance on what to cite in science and engineering.
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