Accuracy Is Not an Issue In Shift to Metric System
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As the United States goes metric and American citizens become familiar with the new units, they are likely to encounter one or both of the following opinions:
- Metric is more accurate than the conventional system of measurement.
- Metric is not as accurate as the conventional system.
Obviously, both statements cannot be true, but before we try to cope with this contradiction lets look at why one or the other can appear to be true:
Metric units may seem more accurate because they have a more scientific basis than our customary “inch-pound” units. For example, a yard was initially defined as the length from the nose of King Henry I of England to the tip of his thumb. One inch equalled “three barley corns, round and dry, taken from the center of the ear, laid end-to-end.” Such definitions imply a lack of uniformity. In contrast, the metric meter was initially defined as one ten-millionth of the quadrant of the earth. (A quadrant of the earth is the distance from the north pole to the equator.) The centimeter is l/100th part of the meter. Incidentally, that means that 100 centimeters make a meter, just like 100 cents make a dollar in our decimal currency. These relations are orderly—and exact.
Illusion of accuracy
Another circumstance may make metric appear more accurate than our conventional system: When dual measurements are used, a metric equivalent is most commonly given for an even inch-pound size, and frequently the metric equivalent is expressed with more digits than necessary. For example, a label might read “3 quarts, 2.839 liters,” giving the impression to some people that metric is more accurate, and also that metric is more complicated.
Some people question the accuracy of the metric system on the following account: Units for the same quantity in the metric and inch-pound systems are not of the same magnitude. Current experience with metric weather forecasting has given some people the impression that temperatures expressed in degrees Celsius (°C), are less exact than when expressed in the more familiar degrees Fahrenheit (°F). This impression results from the fact that 1 degree Celsius is equal to 1.8 degrees Fahrenheit; a 5°C interval is equal to a 9°F interval. This means, in practice, that both 71°F and 72°F have a metric equivalent of 22°C. In a similar manner, the equivalent metric body weight of both 121 pounds and 122 pounds is 55 kilograms.
With these examples in mind, let’s go back to the original question, “Is metric more—or less—accurate?” and examine the facts.
Accuracy is relative
First of all, the accuracy of a measurement system is not based on the standards or definitions for the units that make up that system. In fact, it is technically incorrect to speak of the accuracy of a measuring system. (One measuring system is as applicable as any other, so long as each is coherent.) It is proper to speak of the accuracy of any given measurement, or of the accuracy available from a measuring instrument. Thus, the metric system is not more accurate than our inch-pound system, regardless of how either evolved. Measurements with a meter stick and a yard stick can both have wide degrees of accuracy, depending only on how well the instruments are calibrated and used.
Secondly, conversions between any two systems can make the secondary system appear more accurate and more complicated. (Right now the secondary system is metric.) Such conversions should be avoided, unless necessary for some special reason. When they occur (for example in the dual-dimensioning of products to aid shoppers in making comparisons between new metric sizes and similar items not yet changed), they reflect the fact that even sizes, such as 1 quart, are not so “even” when converted. Eventually, values such as 0.946 liter (or 1 quart) will be changed to “1 liter” (or 1.057 quarts) and the present situation will be reversed.
Importance of intervals
Finally, it is true that the interval between successive numbers for some units, such as temperature (°C vs. °F) and weight (kg vs. lb) is larger in metric than in inch-pound units. Consequently, when temperature or weight is expressed in whole numbers, the metric value covers a wider range than the customary value. In everyday life, this difference is not important. For example, when planning what clothes to wear or deciding whether to go on a picnic, it really doesn’t matter if it is 71°F or 72°F. It is sufficient to know that it is 22°C. We most certainly cannot tell the difference when we go outdoors between 71°F and 72°F, and probably not between 70° F and 73° F, either. Similarly, our body weights change slightly during the day, depending on our level of activity and the amount of food we eat, so it really doesn’t matter that both 121 pounds and 122 pounds are reported as 55 kilograms. There are, of course, cases where temperatures or weights are needed within narrower ranges, as in laboratory experiments. With proper equipment, such measurements can be carried out easily and successfully. Temperatures can be measured to a thousandth of a degree Celsius, or less. Similarly, weight can be determined within a very minute range.
In short, there is no practical difference in the accuracy of the metric and the inch-pound systems. In either, you can measure with the amount of accuracy you need.
Reprinted from Dimensions, June 1978, published by the National Bureau of Standards, U. S. Department of Commerce.
