scientific argument, including whether the author(s) are sufficiently acquainted with recent relevant research that bears on their study, whether the data was collected or considered appropriately and reproducibly, and whether the data discussed supports the conclusion offered and the implications suggested. Novelty is also key: existing work must be appropriately considered and referenced, and new results improving on the state of the art presented. Reviewers are usually unpaid and not a part of the journal staff—instead, they should be "peers", i.e. researchers in the same field as the paper in question. Standards and impact The standards that a journal uses to determine publication can vary widely. Some journals, such as Nature, Science, PNAS, and Physical Review Letters, have a reputation of publishing articles that mark a fundamental breakthrough in their respective fields. In many fields, a formal or informal hierarchy of scientific journals exists; the most prestigious journal in a field tends to be the most selective in terms of the articles it will select for publication, and usually will also have the highest impact factor. In some countries, journal rankings can be utilized for funding decisions and even evaluation of individual researchers, although they are poorly suited for that purpose. Reproducibility and replicability For scientific journals, reproducibility and replicability of the scientific results are core concepts that allow other scientists to check and reproduce the results under the same conditions described in the paper or at least similar conditions and produce similar results with similar measurements of the same measurand or carried out under changed conditions of measurement. Types of articles There are several types of journal article; the exact terminology and definitions vary by field and specific journal, but often include: Letters (also called communications, and not to be confused with letters to the editor) are short descriptions of important current research findings that are usually fast-tracked for immediate publication because they are considered urgent. Research notes are short descriptions of current research findings that are considered less urgent or important than Letters. Articles are usually between five and twenty pages and are complete descriptions of current original research findings, but there are considerable variations between scientific fields and journals—80-page articles are not rare in mathematics or theoretical computer science. Supplemental articles contain a large volume of tabular data that is the result of current research and may be dozens or hundreds of pages with mostly numerical data.