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Measuring and Counting | Forensic Science

Measuring is different from counting. However, in forensic science the two are often suggested to be the same. This needs to change.


Measuring is the assignment of a number, and all the uncertainties of that of that number, to something.  The purpose of assigning a number is to give meaning to the object measured. To give meaning includes:


An object placed upon a scale shows its weight to be 41 pounds. If the object must be less than 50 pounds, then the number produced by the scale indicates it meets this requirement.

However, you must know how far from its true value might the 41 pound number be off by? Uncertainty is the amount of doubt (e.g. the amount of possible variation) you should expect that number might be off.

Fit for Purpose

Now, assume there are two scales. The same object weighing 41 pounds is place on both. However, it was determined that Scale A produces numbers that can be off by as much as 30 pounds. At the same time, it was also determined that the number produced by Scale B merely off by as much as 3 pounds. This difference is significant because you purpose is to determine whether the object is less than 50 pounds (i.e your purpose).

Knowing the amount of uncertainty contained in the number helps distinguish counting from measuring.

Measuring relies upon estimation. The choice of data, the methodologies employed, and level of quality measures used tells you how confident you can be in the estimation. Once you have a reliable estimation of how close a number may be (or not be) to the true value, you can make informed decision as to what purposes the number can be used – and not used.


Counting is a process of determining the number of elements. It is premised on the being able to identify all the things you seek to count.

A Technique

Counting is usually a technique within a measuring process (methodology). Counting can result in an exact number. However, measurement will never claim to represent a true value. Measurements are merely estimations.

Counting a True Value

Counting an exact amount of something is often not possible or practical. The thing you are intending to measure (the measurand), the matrix it is found in, or the level of accuracy required may make counting impossible. Thus a system is needed to provide a reliable estimation which you can rely upon.

Things to Consider

Some things to take into account when making an estimation:

Distinguishing Molecules

Some molecules are so similar to others that it is often impossible continuously distinguish them from each other. Thus, they cannot be easily counted.

Location of Molecules

Some substances are contained in places we cannot practically enter to count them. The best way to know how much alcohol is affecting a person’s brain at a particular time would be to take a sample of brain tissue. However, society has not yet determined such a procedure falls outside the protections of a person’s 4th Amendment rights.

Indirect Measurements

The results of a gas chromatograph are often used to determine whether a person’s alcohol concentration is above a legal limit in DUI cases. However, the machine does not measure a person’s blood alcohol concentration. If properly used, the machine merely counts the number ethanol molecules in a gas portion of a headspace vial. Thus, it indirectly counts a microscopic amount ethanol from a tiny sample.

Why This Matter

In sum, a measurement is much more than counting. It is based upon a machine’s indirect count of a substance which results from combining algorithms, and programmed assumptions of a machine’s software. This is known as an uncertainty calculation.

Counting is what you do to get a number. Measuring is what you do if you want to know the truth about the number. Counting will produce a result, but is may not be fit for your purpose in a court room.

Lawrence Koplow Signature

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