Philippe Theys
Data Quality Manager

Units give sense to numerical data. In the same way as the countries that are members of the European community switched to a single currency, the Euro, at the end of 2001, many attempts have been made over the years to move the world to a single system of units of measurement.

United States Secretary of State Thomas Jefferson asked Congress in 1790 to adopt a decimal system of weights and measures "...and thus bring the calculations of the principal affairs of life within the arithmetic of every man who can multiply and divide plain numbers." A few years later, the French Assembly adopted the metric system to encourage trade within France. Before then, each province had a separate system of units, sometimes using the same name for units representing different quantities. These numerous systems were often of tremendous complexity and ambiguity as shown below:

The metric system was introduced in the last years of the eighteenth century, but it was a struggle to have it completely implemented. Several decades passed before it was fully deployed in France, but it is now used in the whole world except for a handful of countries: Bangladesh, Liberia and The United States. The metric system has been improved to incorporate all aspects of physics and has been renamed Système International.

Système International, through many contributions, has evolved into a totally unambiguous and coherent system. When correctly implemented, it labels physical quantities in clear and well-defined terms. Units are each abbreviated in a unique and unambiguous way: an upper or lowercase letter represents one, and only one, quantity or multiple. For example:

The multiples are^*:

The units may be used alone or in combination with the multiples:

Unfortunately, SI is sometimes used with excessive flexibility and lack of discipline. The SI meaning of M is an abbreviation of mega, the Greek word for "large," representing 1,000,000 or 10⁶, while in the United States, M is often understood as the Roman numeral for 1,000. Another common mistake is to ignore the rule concerning multiples and exponents. The exponent applies to the unit and to the multiple. So km² means km • km or 10⁶m², which is 1,000,000 square meters. It is not k • m • m, which would be10³m², or 1,000 square meters.
These details may sound a bit mundane and the enforcers of SI may seem narrow-minded, but if an organization or individual gets it wrong, it can be very expensive or dangerous.

Here are a few examples of mistakes induced by a poor control of units:

• A national power company suffered from a mix up between prices quoted in kilo-Watt-hour (kWh) and therms. It committed itself to paying US$800,000 for gas worth US$50,000, while trading on the market.
• In Canada, a plane ran out of fuel because the pilot mistook liters for gallons! The passengers were lucky that the captain was better as a glider pilot than he was with units. He landed the plane safely without power on an emergency airstrip.
• More recently, the Mars Climate Orbiter, a NASA spacecraft, swooped in too low as it headed for Mars orbit, dipped too deeply into the atmosphere and was never heard from again. When a NASA contractor told the navigators how much force the thrusters had applied to the spacecraft, they used units of pounds; while NASA assumed the data were in newtons.

So you may find that the answer is 42, but make sure you know what the units are.