Measures and Metrics

A metric is a measure. A metric system is a set of measures that can be combined to form derived measures-for example, the old English system of feet, pounds, and hours. These metrics can be combined to form derived measures as in miles per hour.

Measure has been defined as "the act or process of determining extent, dimensions, etc.; especially as determined by a standard" (Webster's New World Dictionary). If the standard is objective and concrete, the measurements will be reproducible and meaningful. If the standard is subjective and intangible, the measurements then will be unreproducible and meaningless. The measurement is not likely to be any more accurate than the standard. Factors of safety can correct for some deficiencies, but they are not a panacea.

Craft: The Link between Art and Engineering

My great-grandmother was a craftsperson. A craftsperson is the evolutionary link between art and engineering. My great-grandmother made excellent cookies. Her recipes were developed and continuously enhanced over her lifetime. These recipes were not written down; they lived in my great-grandmother's head and were passed on only by word of mouth. She described the steps of the recipes using large gestures and analogies: "mix it till your arm feels like it's going to fall off, and then mix it some more." She guessed the temperature of the oven by feeling the heat with her hand. She measured ingredients by description, using terms like "a lump of shortening the size of an egg," "so many handfuls of flour," "a pinch or this or that," and "as much sugar as seems prudent."

Great-Grandmother's methods and metrics were consistent; she could have been ISO certified, especially if the inspector had eaten any of her cookies. But her methods and metrics were local. Success depended on the size of her hand, the size of an egg from one of her hens, and her idea of what was prudent.

The biggest difference between an engineer and a craftsperson is measurement. The engineer does not guess except as a last resort. The engineer measures. The engineer keeps written records of the steps in the process that he is pursuing, along with the ingredients and their quantities. The engineer uses standard measuring tools and metrics like the pound and the gallon or the gram and the liter. The engineer is concerned with preserving information and communicating it on a global scale. Recipes passed on by word of mouth using metrics like a handful of flour and a pinch of salt do not scale up well to industrial production levels. A great deal of time is required to train someone to interpret and translate such recipes, and these recipes are often lost because they were never written down.

Operational Definitions: Fundamental Metrics

The definition of a physical quantity is the description of the operational procedure for measuring the quantity. For example, "the person is one and a half meters tall." We know from this definition of a person's height by what metric system the person was measured and how to reproduce the measurement. The magnitude of a physical quantity is specified by a number, "one and a half," and a unit, "meters." This is the simplest and most fundamental type of measurement.

Derived units are obtained by combining metrics. For example, miles per hour, feet per second, and dollars per pound are all derived units. These derived units are still operational definitions because the name tells how to measure the thing.

How Metrics Develop and Gain Acceptance

If no suitable recognized standard exists, we must identify a local one and use it consistently-much like my great-grandmother did when making her cookies. Over time, the standards will be improved.

Developing precise and invariant standards for measurement is a process of constant refinement. The foot and the meter did not simply appear overnight. About 4,700 years ago, engineers in Egypt used strings with knots at even intervals. They built the pyramids with these measuring strings, even though knotted ropes may have only been accurate to 1 part in 1,000. It was not until 1875 that an international standard was adopted for length. This standard was a bar of platinum-iridium with two fine lines etched on it, defining the length of a foot. It was kept in the International Bureau of Weights and Measures in Sevres, France. The precision provided by this bar was about 1 part in 10 million. By the 1950s, this was not precise enough for work being done in scientific research and industrial instrumentation. In 1960, a new standard was introduced that precisely defined the length of the meter. The meter is defined as exactly 1,650,763.73 times the wavelength of the orange light emitted by a pure isotope, of mass number 86, of krypton gas. This standard can be measured more accurately than 1 part per 100 million.

Once a standard is introduced, it must still be accepted. Changing the way we do things requires an expenditure of energy. There must be a good reason to expend that energy.