Why do some people say that evolution is \u2018only a theory\u2019 when there\u2019s so much evidence to support it? In short, they don\u2019t understand how the word \u2018theory\u2019 is used in science. We see similar problems with \u2018hypothesis\u2019 and \u2018significant\u2019 in scientific writing, where these terms have meanings that don\u2019t quite fit their everyday usage. As such, we\u2019ve prepared this guide to the scientific uses of \u2018theory\u2019, \u2018hypothesis\u2019 and \u2018significant\u2019.\nWhat Is a Theory?\nIn day-to-day life, we often use the word theory to mean an \u2018idea\u2019, usually something unproven. For example, if a friend or family member says:\nI have a theory about why sharks look so angry.\nYou would probably assume that they\u2019ve not based their theory on a lot of scientific research (unless they\u2019re a shark scientist).\n\n[caption id="attachment_4809" align="aligncenter" width="377"] You can always spot a shark scientist from the bite marks.[\/caption]\n\nHowever, a scientific theory is an idea that has been tested repeatedly without being disproved. Consequently, there\u2019s no such thing as \u2018only a theory\u2019 in science!\nWhat Is a Hypothesis?\nA theory starts out as a hypothesis. This term is less common in everyday life, but we do sometimes hear it used to refer to an assumption or an educated guess.\n\nThis is not far from its technical use, but a \u2018scientific hypothesis\u2019 is something quite specific:\n\n \tIt should attempt to explain something\n \tIt should be testable\n\nThe second point here is crucial for how a scientific hypothesis is phrased. For example, we could say:\nPlants grow more quickly if you play them heavy metal music.\nThe methodology section of the paper would then explain how to test this. And once the results are in, we would either accept or reject the hypothesis.\n\n[caption id="attachment_4830" align="aligncenter" width="343"] After extensive testing, it turns out that daisies prefer drill'n'bass.(Photo: fietzfotos)[\/caption]\nWhat Does \u2018Significant\u2019 Mean?\nIn daily life, \u2018significant\u2019 means important or noteworthy. The opposite is \u2018insignificant\u2019, which means \u2018unimportant\u2019. But in scientific writing, \u2018significant\u2019 usually refers to statistical significance.\n\nIf a test result is \u2018statistically significant\u2019, it is unlikely to be due to chance. If the result could have been influenced by random variations, we say it is \u2018non-significant\u2019 (not \u2018insignificant\u2019).\n\n[caption id="attachment_4831" align="aligncenter" width="415"] Significance is determined with help from a 'p-value'.(Image: Repapetilto\/wikimedia)[\/caption]\n\nCrucially, something can be statistically significant without being noteworthy. If we tested whether beer causes drunkenness, for example, we\u2019d achieve high significance levels during our statistical analysis (thanks, alcohol!). However, nobody would say this is an \u2018important\u2019 insight.\nOther Scientific Terms\nIn this post, we\u2019ve focused on three common terms that have specific meanings in scientific writing. However, there are other words that have specialised uses. If a physicist talks about the \u2018flavour\u2019 of a quark, for example, she won\u2019t be talking about how it tastes.\n\n[caption id="attachment_4832" align="aligncenter" width="306"] Unless she's talking about this kind of 'quark'. We warned you that this can be confusing!(Photo: SKopp\/wikimedia)[\/caption]\n\nThis shows why you need to be careful with scientific terminology. If you see a familiar word used in an unusual way, look up what it means in the sciences. And don\u2019t forget that having your work proofread by an expert can help!