Measurement is the process of associating numbers with physical quantities and phenomena. It is one of the most fundamental aspects of science, and it’s what distinguishes real scientific theory from pseudoscience. It is also critical to commerce, engineering, construction, and almost all everyday activities. The elements, conditions, limitations, and theoretical foundations of measurement have been the subject of much study.

A basic definition of measurement involves comparing **what measurement is** a physical quantity to a standard or accepted pattern, and assigning it a numerical value. This number can then be used to compare objects, quantify a characteristic of an object, or make a decision about the characteristics of that object. The process of measuring requires a measuring instrument that is carefully calibrated to produce an accurate result. The most common measuring instruments include rulers, tape measures, calipers, and voltmeters. In some cases, surveyors use a variety of different measurement tools in order to gather information about the environment.

In addition to a measurement instrument, a good measurement requires a method for recording the results of the measurements, and a system for making evaluations of the accuracy and precision of the measuring instrument. There are several ways to assess the accuracy of a measurement, including measuring it again, checking for systematic errors in the measuring instrument, and comparing results with available references.

There are several levels of measurement, including nominal, ordinal, interval, and ratio scales. A nominal scale uses numbers to label and categorize data, but does not indicate the degree of variation between each category. An example of a nominal scale would be the color swatches of a shirt, where each swatch has its own name but does not distinguish between “very blue” and “much more blue.”

An interval measurement scale indicates the amount of change in the quantity being measured. This type of scale allows comparisons to be made between data points, and addition and subtraction are appropriate. The Fahrenheit temperature scale is an example of an interval measurement scale.

Ratio measurements allow us to describe the relationship between two quantities, and are often used when describing time and space. The speed of a car, for instance, is often described as “a mile per hour” or “a minute per second.”

Percentages are a familiar measurement tool that can be used in many contexts. For example, a school may report student grades as percentages. To find a student’s percentile, the grade is divided by the total number of grades in that class, then multiplied by 100 to give a percentage value.

A metric unit of measurement is the kilogram (kg), and it is often abbreviated as kg or g. The kilogram is the base unit of measurement for most metric systems, and it is a relatively simple unit to use since there are no complicated conversions involved. The metric system is widely used throughout the world. Units of measurement are based on historical agreements among scientists over the years. For example, a meter is as long as it is because that is what scientists agreed to use as a base unit of length many years ago.