An old saying goes that a man with one watch knows what time it is, but a man with two watches is never sure. Scales must contend with a similar conundrum: If you weigh the same object twice, do you get the same result? How about on different days, under changing weather conditions? If so, can the scale remain consistent despite wear and tear?
These factors -- respectively known as repeatability, reproducibility and adjustment -- are all important, but a prospective buyer must also consider more basic questions, such as the scale's weight range, including its minimum and maximum capacity [source: AWS].
Then again, those statistics matter little if a device lacks the necessary accuracy and precision for the job. Although the two terms are often misused as synonyms, they're not interchangeable: Accuracy describes how closely an instrument measures an actual or expected value, whereas precision refers to how granular or exact that measurement is. A speed gun that clocks a 90.4213 mph fastball at 90 mph is accurate but not precise; a speed gun that clocks the same fastball at 88.3246 mph is precise but not accurate.
Scales usually express accuracy as a percentage of full scale (FS) -- the total calibrated range a device can measure. For example, a 1 percent FS accuracy can mean +/- 5 pounds for a 500-pound scale, but +/- 1 pound on a 100-pound scale. Alternatively, error can be expressed as percentage of actual reading (AR): For an object weighing 100 pounds on such a scale, a 2 percent AR would mean an error of 2 pounds, whether the scale had a range of 10 pounds or 10,000 [sources: AWS; Busch; Plint].
The precision of a scale is similarly variable. By industrial standard, regardless of capacity, scales are split into a maximum of 10,000 divisions: Hence a 10,000-pound scale will divvy into one-pound increments, whereas a 500,000-pound scale will divide into 50-pound increments [sources: AWS; Crowley; Mashaney]. Weighing devices are grouped into categories based on these divisions (see sidebar).
With this in mind, always try to select a scale that is has a high enough capacity to handle what you are measuring, but not much higher. This will ensure you get the most precision possible, pound for pound.