Explain xkcd: It's 'cause you're dumb.
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Computer file names often end in file extensions like ".ppt" or ".exe". These extensions are a holdover from early operating systems like DOS in which filenames had a maximum eight characters followed by a period and the three-character extension. The extension was used by the operating system to determine filetype so that the system would know how to handle the file (e.g. which program could open the file). Newer operating systems and file systems now accept longer-than eight-character filenames, and extensions of greater than three characters; although most extensions remain three characters.
Most extensions are created as proprietary to certain pieces of software, although software by other developers may later be designed to be able to read the format (for example, .doc is a Microsoft Word document, although because of that software's popularity, many word processors include the ability to open .doc files). Some common file extensions are not proprietary to a piece of software and may be handled by various programs (.jpg or .gif images are one example). In either case, a file's extension is generally a good indicator of what type of data the file contains.
Certain file types are more prevailant for certain uses, with some being almost exclusive to one use, while other are in general use and might contain almost anything. Here, Randall presents a series of file extensions which often contain information, and he is rating the reliability of the information they generally contain from most reliable to least.
- .tex files are source files for the programs TeX and LaTeX, which are used often and almost exclusively by academics, especially in mathematics and the hard sciences. .tex pretty much means serious business, and Randall does not anticipate that anyone would use such a format other than for reliable information.
- .pdf files are a document format by Adobe, frequently used for publication. pdf files are commonly used by professionals and companies for official documentation. Thus, a .pdf file is likely to be some type of final product or polished work.
- .csv are comma-separated values: tables of information delimited by commas, and are often computer-generated (from, say, a scientific experiment).
- .txt files contain only plain text, no "rich text" or anything fancy. Programmers often use them for README files.
- .svg files are a vector graphics format used a lot for diagrams, such as on Wikipedia.
- .xls and .xlsx files are spreadsheets used and created by the program Microsoft Excel, part of a bundle of applications known as Microsoft Office (also supported by compatible free software such as LibreOffice). These applications are very commonly used, especially for business, finance and data analysis tasks. .xls is used for Excel versions prior to 2007, while .xlsx is used for Excel versions 2007 and later.
- .doc files are a rich-text document format used and created by the program Microsoft Word, another application in the Microsoft Office bundle. As with .xls, almost anyone with access to Microsoft Office could easily make one of these. While Excel is generally used for creating tables and presenting data, Word could be used for any text-based document. Thus, Word documents tend to be far more prevalent and casually created than Excel documents, which is presumably why Randall doesn't trust it much.
- .png files are a bitmap image format designed for the Internet. They enjoy wide popularity for providing crisp, full-color images with lossless (invisible) compression. Almost all xkcd comics, this diagram included, use PNG. But since he rates the format so low, is Randall saying we shouldn't trust this chart?
- .ppt files are used and created by the program Microsoft PowerPoint; as with the other two Office applications, almost anyone could easily make one of these. As they are usually used for presentations rather than documents, the information in them may be arranged differently, possibly to "dumb down" the content, or in marketing materials or talks in which the author may not be very objective. further, several years ago, PowerPoint presentations were sometimes included instead of plain images as attachments in e-mail forwards containing inaccurate information. These emails still occasionally circulate, and may be the source of Randall's distrust.
- .jpg files are another image format with high compression capabilities, good for storing photos, and not so good for many other things. JPEG is prone to annoying compression artifacts, so it's generally bad for images of numerical or textual information. Cameras often take high-quality photos in the .raw format and then compress it into a much, much smaller JPEG; a JPEG file's content may be less trustworthy as it doesn't contain as much of the detail in the original image.
- .jpeg files are the same thing as .jpg files; however, it is more likely that an image with this extension was created manually rather than automatically by, for example, the aforementioned digital camera. This makes it less trustworthy.
- .gif files are yet another bitmap image format, notable for supporting short animations. GIF was once the Internet image file format until PNG gradually replaced it. Since GIF is the only common image format capable of animation, it is often used to contain things like silly clips of cats falling into boxes, or blinking advertisements claiming that you're the 570,000th VISITOR!. Their frequent use in the most annoying types of advertisements (pop-ups, etc.) is presumably why Randall deems it the least trustworthy out of all file extensions listed here. Additionally, GIF files can be concatenated with a ZIP file to hide an archive within a GIF file (Steganography), thus resulting in a file that can easily be untrustworthy.
Note that while the extensions .xls/.xlsx, .doc, and .ppt were originally exclusive only to Microsoft Office and users of Windows, there now exist a number of open source programs such as Open Office, Libre Office, and some Android apps that are capable of editing such files. These programs can run on systems other than just Windows, such as Linux, perhaps contributing to making them even more widespread and easy to make than before.
The title text refers to how .txt files contain only plain text and nothing else, meaning that any alignment (such as for indentation or tables) would have to be performed manually by adding in spaces or tabs. Anyone who would go through such an effort to improve their text's readability is likely to be trustworthy.
- Trustworthiness of Information by File Extension
- [A bar graph charting this. No units or figures are given, but for ease of comprehension this transcript will arbitrarily designate the highest score as "+100"; subsequent scores are estimates based on the size of their bars.]
- .tex: +100
- .pdf: +89
- .csv: +85
- .txt: +67
- .svg: +65
- .xls/.xlsx: +49
- .doc: +21
- .png: +15
- .ppt: +14
- .jpg: +3
- .jpeg: -8
- .gif: -36
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