Accuracy is the Enemy of Precision: Part I

An aphorism that I have been hearing frequently is: “accuracy is the enemy of precision”. When I was gainfully employed (I am now retired) I did research at a drug company and also at the EPA.  I dealt every day with the concepts of accuracy and precision.  The reality is that one cannot divide these into separate concepts because they overlap to a very large degree.  A more correct aphorism might be “Accuracy is the Friend Precision”, or alternatively “Without Accuracy There is No Precision” or even “Without Precision There is No Accuracy”.  The most unsettling aspect of this aphorism is that it posits accuracy and precision as “enemies” suggesting that one would definitely want one of these but not the other, that one is good and one is bad, or that you should strive for one but not the other.  Anytime you have a broad concept reduced to a single statement, it will always be so that part of the population will interpret this literally, and part of the population will look behind it, with a grain of salt.  An over-the-top example, yet starkly demonstrative, is the campaign slogan for Barry Goldwater a 60s presidential race was “extremism in the defense of liberty is no vice”.   A lot of people took that completely at face value (my parents certainly did).  Others wondered what “extremism”, “defense”, “liberty” and “vice” actually meant.  I just remember being scared by the statement.

There are aphorisms and there are truisms. Truisms are a subset of aphorisms.  An aphorism is a pithy statement, meant to be applied broadly.  Because it is succinct, and generally sounds like it might be true, it appears to state a profound truth.  And it may in fact do so.  The aphorism in question above sounds like it is, or might be, based on facts, but it is not.  Anytime just 6 words are used to define a set of concepts that people have pondered and studied and written libraries of books about, there is the potential for a great disservice.  On the other hand, a truism is a statement that is actually based on facts, and on careful study.  Again, it is simplistic, but its value is that it is cautionary as well.

I have both an aphorism and a truism that I use a lot. “Good News Will Wait, Bad News Hunts You Down!”  This is an aphorism and actually relates more to the psychological impact of good and bad news.  However it t is written to imply that the speed of news is related to the quality of said news. If you think about it, you will realize that good news is usually a phone call or an email away.  Bad news (notice of audit from the IRS for example) is usually a registered letter away (3 days).  So the aphorism is not literally true but is does resonate at a deeper level.  A truism that I use is “GIGO”.  This is apparently a computer programming truism (Garbage In, Garbage Out) and particularly relates to database management.  However it can be applied broadly to mean that the results you get are directly related to the effort you are willing to apply to achieve them.  This is in fact so true, that it actually goes without saying.

So back to accuracy and precision. Some definitions.  Accuracy can be defined as the ability to consistently achieve a desired result related back to a defined external standard.  For example a measure related back to the length of a bar of platinum on a pillow in France, or the oscillations per second of an electronic cloud around an atom.  Precision is defined as the ability to get the same result time after time.  Accuracy means that you will achieve a desired result (absolutely correct); precision simply means that you will achieve a consistent result, which may or may not be accurate.  It goes without saying that you cannot be accurate without being precise, but you can be precise but not accurate.

So how does this relate to woodworking? You might say that a ruler is an external standard.  In theory all measurements of time and dimension relate back through a series of secondary standards to a root primary standard (again, a bar of platinum in France for example).  A common example of an external standard for woodworkers is a cut list with dimensions listed.  Cut lists can be dangerous in that they do not encourage adaptation of a project to meet specific needs.  More importantly, they rely on the concept of GIGO (you assume that the cut list is correct), and then on the accuracy of the maker to replicate the dimensions of the cut list.  On the other hand, a cutlist can be essential if one is attempting to (precisely/accurately?) duplicate an object.  You could even say that the object is the external standard.  If you desire to duplicate an object, would you strive for accuracy or just precision?

Where I teach, we rely on several systems of measurement (not one of which involves a ruler): the dimensions as related to the purpose of what we build (tool boxes are dimensioned to fit the tools we want in them), the dimensions of the stock as references within the project (opposite sides of a box must match), the dimension of the tools we will use in the job (the space between dovetails defined by the chisel we will use to cut that space), and then sometimes a story stick.

What is a story stick if not a form of a ruler? In actuality a story stick is just one step removed from a ruler.  It is a ruler without numbers.  If we do not use numbers, does this mean we are not being accurate?  The job of a story stick is to enhance precision (marking the same dimension time after time) and to ensure accuracy (the mark on the story stick is accurately based on a predetermined standard).  Using a story stick does not mean that you have separated yourself from applying accuracy or precision in your work.  You just don’t see the numbers anymore, but they are embedded in the stick nonetheless.  It would be facetious to say that with a story stick you only apply precision, but not accuracy.

When I build a box, if I want it to be a rectangle/square shape, then opposite sides must have the same dimensions. This goes without saying.  To do so, I generally don’t measure the lengths of both opposite parts.  I measure one part, square it up, then make the second part match the first.  And I do so by finger touch.  The first piece is accurate, the second piece is precise.  It is absolutely important that the two pieces match, but not necessarily that they be an exact dimension (within limits).   In other words, I shoot for a dimension, but recognize that after squaring up, etc., I may no longer have that (accurate/exact) dimension.  I am both accurate and precise, within whatever limits I set for my work.  Since I have measured the first piece, but now it is a slightly different dimension, can I say that I have been only precise but not accurate?

This leads me to the second part of this discussion, the role of error in accuracy and precision.

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