Watch accuracy & precision
Watch accuracy and precision a confusing topic

We can start improving communication by clarifying the concepts behind three frequently-used terms in watchmaking: accuracy, adjustment, and regulation. Accuracy in watchmaking does not carry the meaning that is widely accepted elsewhere. Many of us add to the confusion on accuracy by using the words adjustment and regulation interchangeably. Its time to tidy up our language.

In the following, I will review the meaning of these basic terms in watchmaking, their conventional usage in science and engineering, and suggest future usage that could help reduce misunderstanding amongst eclectic members of TimeZone. To paraphrase Mark Twain, the difference between the right word and the wrong word may be the difference between a fire and...a firefly.

My objectives are to encourage people to 1) reserve the word "accuracy" for time-keeping, 2) use the word precision for consistent performance, 3) recognize that precision--resulting from adjustment--is the real basis for quality, 4) pay attention to the difference between adjustment and regulation, and 5) have a realistic expectation on what mechanical watches can do.


1) Rate stability in a single positionThis is a measure of how the instantaneous rate fluctuates with time, for a movement in a stationary and single position. This rate has nothing to do with a time standard (accuracy). Walt pointed out that a high-quality escapement is essential to producing a stable rate, the foundation of precision (or consistency).

2) Average rate in a single positionThe rate is averaged over a short period of time--say five minutes--and repeated at time intervals of less or more than a day. These average rates should vary little for a movement that is well-designed, and built with high-quality parts--especially the balance assembly.

3) Average rate in different positionsThis is the comparison of the different average rates, each for a positions of the watch (dial up/down, crown up/down/left/right). As Walt pointed out to me, if power to the escapement is inadequate or excessive, these rates depend on the main spring and gear train. Otherwise, these rates depend on the poise of the balance wheel, and adjustments to the hairspring and regulator. This adjustment yields precision (traditionally called accuracy in watchmaking), i.e., consistent rates that are essential to achieve accuracy. in different positions.

4) Absolute rate Absolute rate refers to the time keeping of the watch relative to a standard (electronic or atomic clock). Matching this absolute rate to a reference (by regulation) is relatively easy if the average rates are stabilized by adjustment. I must point out here my discomfort with the word absolute. It is normally reserved for a reference, or a hypothetical and immutable value, non of which really applies to a watch rate.


Adjustment and regulation are the two stages of watch timing. Adjustment, the more complex of the two, ensures a consistent rate over time and position. Regulation, which follows adjustment, is a relatively simple step that yields accuracy.

ADJUSTMENT is the process of fine-tuning the balance wheel, hairspring, and regulator to stabilize the watch rate over time (in a single position) and to consistently produce similar rates in different positions. Complex and time-consuming, adjustment is usually done at the factory. As mentioned earlier, these rates also depends on the quality of the main spring, gear train, and escapement. After adjustment, the rate of the watch still has a residual variation, which is small for a high-quality movement.

REGULATION is the process of matching the (absolute) rate of the movement to a reference rate (electronic or atomic clock). Regulation improves accuracy in the strictest sense of the word (see below), referred to as time-keeping in watchmaking. It is a relatively simple procedure that entails changing the effective length of the hairspring via the regulator, or changing the center of mass of the balance wheel to change the absolute rate. Accuracy still varies in actual use, as a watch is never perfectly adjusted, so further regulation may be needed after purchase or service to suit the users life-style.

In summary, adjustment renders a watch rate as insensitive as possible to the rigors of normal use. Regulation is a simpler process that matches the watch rate to the rate of a reference clock.


Now lets turn our attention to the much abused term on TimeZone, accuracy, and its often neglected but more important twin, precision, and see how they relate to adjustment and regulation.

ACCURACY is by definition conformance to a standard. Something is accurate when the difference (in time or distance) from a reference is small. So strictly speaking, a watch is accurate when it keeps time with respect to a reference clock, the same way that a shooter is accurate when his bullet hits the bulls-eye, or close to it, to borrow Graiches analogy. In practice, we say a watch is accurate when its average daily rate (or absolute rate) is very close to the rate of an electronic or atomic clock. For example, a watch may gain 5 seconds a day compared to the reference: its daily rate is accurate to +5 seconds.

Unfortunately, in watchmaking, the ability to match a reference clock is actually called time-keeping! Accuracy has a different meaning: it refers to the consistency or reproducibility of the watch rates (between time periods, and between position). This usage actually violates the definition of the word accuracy, and runs counter to its usage in science, engineering, and other fields as well. Accuracy must relate to a comparison with a reference, not to consistency or reproducibility, which is strictly the domain of precision. This raises the question on what to do to avoid confusion when people of different experience communicate about watches.

PRECISION is by definition the repeatability or consistency of a process. It tells you how accuracy varies as the measurements are repeated over a range of parameters--time and positions for watches. Imprecision, the opposite, represents the amount of scatter in the results. A shooter is precise when his bullets hit roughly the same spot on the target every time--a tight grouping or small scatter--even if this spot is far off the bulls-eye. How far that spot is from the bulls-eye is the measure of accuracy, by definition.

So, precision describes the rate changes as the variables (time, position) change: it is the product of adjustment. Accuracy describes time matching with a reference: it is the product of regulation. A watch that gains 1 second per day is very accurate, but as Walt Odets has pointed out many times, this accuracy may simply be a fortuitous accident, not a reliable sign of quality. The same watch may gain or lose tens of seconds within the same day; its daily rate may also change from day to day, from person to person. It is clear then that accuracy is meaningless without precision as an unadjusted watch cannot be trusted to run with the same accuracy consistently. But precision, the foundation of high quality, is better appreciated when accompanied with accuracy.


Most watch consumers equate daily rate (strict accuracy) with quality. This attitude is simplistic and perhaps naive, though completely understandable. After all, how well a watch keeps time is an important and practical reason for buying it. Thus, good design, high-quality parts, and the craft of adjusting that produces great precision go mostly unappreciated, never mind that without precision accuracy is impossible, or simply an accident. Its hard indeed to convince a person that an adjusted watch that gains +20 seconds per day, consistently to 1 second, is far superior to an unadjusted watch that gains +2 seconds a day with a fluctuation of 10 seconds. Though true, its a hard sell. The emotional impact of an accurate daily rate on a consumer can be overwhelming.

I got an appreciation the importance of adjustment and precision after my IWC Mark XII and IWC Porsche Design Compass watch (ETA) were serviced. While the daily rates--actual time-keeping on my wrist--of both watches were nearly the same after service, about +3 seconds per day, the Mark XII, which was adjusted to 5 positions, showed rates that differ by less than a second in all positions, a breath-taking performance! This watch will show about the same daily rate on another persons wrist even if his life style differs greatly from mine. By contrast, the ETA in different positions varied by as much as 8 seconds, which was still very good, but clearly inferior to the adjusted JLC movement. The +3 second daily rate on my wrist is fortuitous. If I left the PD in a different position--I did--or sold the watch to someone else--I wont--the daily rate would change noticeably.


I feel that the best way to prevent further confusion is to adopt the definitions that are semantically correct and widely used in science, engineering, and other fields. By definition, accuracy is strictly conformance to a standard, and precision describe the consistency or repeatability of a process. Then, the word "accuracy" should be used strictly for good time keeping, a measure of how well a watch (e.g., its daily rate) matches a reference clock. Precision--not accuracy--should be used to describe the consistency or reproducibility of the watch rates. These definitions for accuracy (good time-keeping) and precision (consistent rates) are clear, semantically correct, and in complete agreement with the world outside watchmaking.

We should also make an effort to use the word regulation--not adjustment--for the simple correction of the watch daily rate, usually as part of service. Regulation simply means turning the regulating screw (usually marked with + and -). Adjustment is a vastly more complex process of fine-tuning the balance wheel, hairspring, and regulator to stabilize the watch rates between different time periods in a single position, and between different positions. These rates could also be a function of the mainspring, gear train, and escapement. Adjustment to several positions is usually performed to a very small tolerance on high-quality watches. It yields good precision (consistent rates), without which accuracy (good time-keeping) is impossible, or just a lucky shot. There are very "accurate" mechanical watches out there that are accomplishing their accuracy through the chance averaging of huge positional errors. They're still lousy watches.

Thus, it is important in future discussion of watch performance to talk about not just the daily rate (accuracy), but also how it varies from day to day, and in different positions (precision) to provide a more complete picture of the watch quality. In so doing, we show our appreciation of the real craft in watchmaking: adjustment.

As to our continued obsession with a vanishingly small accuracy in daily rate, In 24 hours, the escapement of a mechanical watch pushes the gears 432,000 times. Since a day has 86,400 seconds, even a watch that runs five minutes fast or slow each day has an accuracy of over 99.6 percent! A finer mechanical watch that gains or loses about nine seconds a day or about a minute a week has a breathtaking precision of over 99.99 per cent. This is very high precision, given the fact that the movement is constantly affected by the earth's gravity, metal expansion and contraction, temperature variations, subtle changes in lubrication and friction, shocks, and so on.