Editing Measurement (section)

== Units and systems ==
{{Main|Unit of measurement|System of measurement}}
A unit is a known or standard quantity in terms of which other physical quantities are measured.[[File:MetricImperialUSCustomaryUnits.jpg|thumb|upright|A [[baby bottle]] that measures in three [[Units of measurement|measurement systems]]—[[metric system|metric]], [[Imperial unit|imperial (UK)]], and [[United States customary units|US customary]]]]

=== Imperial and US customary systems ===
{{Main|Imperial and US customary measurement systems}}
Before [[SI unit]]s were widely adopted around the world, the British systems of [[English unit]]s and later [[imperial unit]]s were used in Britain, the [[Commonwealth of Nations|Commonwealth]] and the United States.  The system came to be known as [[U.S. customary units]] in the United States and is still in use there and in a few [[Caribbean]] countries.  These various systems of measurement have at times been called ''foot-pound-second'' systems after the Imperial units for length, weight and time even though the tons, hundredweights, gallons, and nautical miles, for example, have different values in the U.S. and imperial systems. Many Imperial units remain in use in Britain, which has officially switched to the SI system, with a few exceptions such as road signs, where road distances are shown in miles (or in yards for short distances) and speed limits are in miles per hour. Draught beer and cider must be sold by the imperial pint, and milk in returnable bottles can be sold by the imperial pint. Many people measure their height in feet and inches and their weight in [[Stone (imperial mass)|stone]] and pounds, to give just a few examples. Imperial units are used in many other places: for example, in many Commonwealth countries that are considered metricated, land area is measured in acres and floor space in square feet, particularly for commercial transactions (rather than government statistics). Similarly, gasoline is sold by the gallon in many countries that are considered metricated.

=== Metric system ===
The [[metric system]] is a decimal [[system of measurement]] based on its units for length, the metre and for mass, the kilogram. It exists in several variations, with different choices of [[Units of measurement#Base and derived units|base units]], though these do not affect its day-to-day use. Since the 1960s, the International System of Units (SI) is the internationally recognised metric system. Metric units of mass, length, and electricity are widely used around the world for both everyday and scientific purposes.

==== International System of Units ====
The [[International System of Units]] (abbreviated as SI from the [[French language]] name ''Système International d'Unités'') is the modern revision of the [[metric system]]. It is the world's most widely used [[system of units]], both in everyday [[commerce]] and in [[science]]. The SI was developed in 1960 from the [[metre–kilogram–second]] (MKS) system, rather than the [[centimetre–gram–second]] (CGS) system, which, in turn, had many variants. The SI units for the seven base physical quantities are:<ref>{{SIbrochure9th}}</ref>
{| class="wikitable"
|-
! Base quantity
! Base unit
! Symbol
! Defining constant
|-
| [[time]]
| [[second]]
| s
| hyperfine splitting in [[caesium-133]]
|-
| [[length]]
| metre
| m
| [[speed of light]], ''c''
|-
| [[mass]]
| [[kilogram]]
| kg
| [[Planck constant]], ''h''
|-
| [[electric current]]
| [[ampere]]
| A
| [[elementary charge]], ''e''
|-
| [[temperature]]
| [[kelvin]]
| K
| [[Boltzmann constant]], ''k''
|-
| [[amount of substance]]
| [[mole (unit)|mol]]
| mol
| [[Avogadro constant]], ''N''<sub>A</sub>
|-
| [[luminous intensity]]
| [[candela]]
| cd
| [[luminous efficacy]] of a 540&nbsp;THz source, ''K''<sub>cd</sub>
|}

In the SI, base units are the simple measurements for time, length, mass, temperature, amount of substance, electric current and light intensity. Derived units are constructed from the base units: for example, the [[watt]], i.e. the unit for power, is defined from the base units as m<sup>2</sup>·kg·s<sup>−3</sup>. Other physical properties may be measured in compound units, such as material density, measured in kg·m<sup>−3</sup>.

===== Converting prefixes =====
The SI allows easy multiplication when switching among units having the same base but different prefixes. To convert from metres to centimetres it is only necessary to multiply the number of metres by 100, since there are 100 centimetres in a metre. Inversely, to switch from centimetres to metres one multiplies the number of centimetres by 0.01 or divides the number of centimetres by 100.

=== Length ===
{{Main|Length measurement}}
{{See also|List of length, distance, or range measuring devices}}
[[File:CarpentersRule.png|thumb|right|A two-metre carpenter's [[ruler]]]]

A [[ruler]] or rule is a tool used in, for example, [[geometry]], [[technical drawing]], engineering, and carpentry, to measure lengths or distances or to draw straight lines.  Strictly speaking, the ''ruler'' is the instrument used to '''rule''' straight lines and the calibrated instrument used for determining length is called a ''measure'', however common usage calls both instruments ''rulers'' and the special name ''straightedge'' is used for an unmarked rule.  The use of the word ''measure'', in the sense of a measuring instrument, only survives in the phrase ''tape measure'', an instrument that can be used to measure but cannot be used to draw straight lines.  As can be seen in the photographs on this page, a two-metre carpenter's rule can be folded down to a length of only 20 centimetres, to easily fit in a pocket, and a five-metre-long tape measure easily retracts to fit within a small housing.

=== Time ===
{{Main|Time}}

Time is an abstract measurement of [[elemental]] changes over a non-spatial continuum.  It is denoted by numbers and/or named periods such as [[hour]]s, [[day]]s, [[week]]s, [[month]]s and [[year]]s. It is an apparently irreversible series of occurrences within this non spatial continuum.  It is also used to denote an interval between two relative points on this continuum.

=== Mass ===
{{Main|Weighing scale}}

''Mass'' refers to the intrinsic property of all material objects to resist changes in their momentum.  ''Weight'', on the other hand, refers to the downward force produced when a mass is in a gravitational field. In [[free fall]] (no net gravitational forces) objects lack weight but retain their mass. The Imperial units of mass include the [[ounce]], [[pound (mass)|pound]], and [[ton]]. The metric units [[gram]] and kilogram are units of mass.

One device for measuring weight or mass is called a weighing scale or, often, simply a "scale". A spring scale measures force but not mass, a balance compares weight; both require a gravitational field to operate. Some of the most accurate instruments for measuring weight or mass are based on load cells with a digital read-out, but require a gravitational field to function and would not work in free fall.

=== Economics ===
{{Main|Measurement in economics}}
The measures used in economics are physical measures, [[nominal price]] value measures and [[real price]] measures. These measures differ from one another by the variables they measure and by the variables excluded from measurements.

=== Survey research ===
{{Main|Survey methodology }}
[[File:EMMA experiment.jpg|thumb|Measurement station C of EMMA experiment situated at the depth of 75 meters in the [[Pyhäsalmi Mine]]]]
In the field of survey research, measures are taken from individual attitudes, values, and behavior using [[questionnaire]]s as a measurement instrument. As all other measurements, measurement in survey research is also vulnerable to [[Observational error|measurement error]], i.e. the departure from the true value of the measurement and the value provided using the measurement instrument.<ref>{{cite book | last = Groves | first = Robert | title = Survey Methodology | publisher = Wiley | location = New Jersey | year = 2004 | isbn = 9780471483489 | url = https://archive.org/details/surveymethodolog00robe | url-access = registration}} "By measurement error we mean a departure from the value of the measurement as applied to a sample unit and the value provided. "  pp. 51–52 .</ref> In substantive survey research, measurement error can lead to biased conclusions and wrongly estimated effects. In order to get accurate results, when measurement errors appear, the results need to be [[Correction for attenuation|corrected for measurement errors]].

=== Exactness designation ===
The following rules generally apply for displaying the exactness of measurements:<ref>[https://books.google.com/books?id=CwsBn-r1b6MC&pg=PT41 Page 41] in: {{cite book | last=VanPool | first=Todd | title=Quantitative analysis in archaeology | publisher=Wiley-Blackwell | publication-place=Chichester Malden | year=2011 | isbn=978-1-4443-9017-9 | oclc=811317577 }}</ref>
* All non-0 digits and any 0s appearing between them are significant for the exactness of any number. For example, the number 12000 has two significant digits, and has implied limits of 11500 and 12500.
* Additional 0s may be added after a [[decimal separator]] to denote a greater exactness, increasing the number of decimals. For example, 1 has implied limits of 0.5 and 1.5 whereas 1.0 has implied limits 0.95 and 1.05.