At the top of this pyramid happen to be systematic meta-analyses and opinions, while in the bottom are usually observational analyses and case reports. Often used to study and test hypotheses concerning the nature of the world, a scientific experiment creates controlled conditions which are manipulated to produce measurable and repeatable leads to confirm or disprove an initial theory... http://pandora.nla.gov.au/external.html?link=https://aqueduct-translations.org