Accuracy vs. Precision in Chemistry

Accuracy vs. Precision

NToday we will briefly address the differences between Accuracy and Precision.

These two terms are NOT interchangeable, so it’s best if we start by defining them below…

Accuracy

= how well a measurement agrees with the true value.

= If a measurement is accurate, it agrees with the true or accepted value.

Precision

= how well a series of measurements agree with each other, regardless of what the true value is.

= If a series of measurements are precise, their values “agree” with each other.

— — — — —

In order to get a good understanding of the differences between accuracy and precision in chemistry, let’s go over four dartboard analogy situations below…

Dartboard Analogy for Accuracy and Precision

A dart represents the results of a measurement or experiment. The bulls-eye represents the true value of the measurement or experiment.

Examples of Accuracy vs. Precision

Left dartboard

The results shown in the left dartboard above (the 5 X’s) are neither accurate nor precise.

There’s a lot of random errors in our technique (human errors / mistakes), so much so that it’s too difficult to tell if we have any systematic errors (instrumentation errors.)

Middle dartboard

The results shown in the middle dartboard in the above image (5 X’s) are precise, but not accurate.

Very few random errors are being made (if any), but something is not quite right with our procedure, or our measurement devices, etc.

We are experiencing only systematic error (instrumentation error).

Right dartboard

Good job! Our results are both precise and accurate.

No random errors, and no systematic errors. In other words, no human errors and no instrumentation/machine errors :-)

— — — — —

QUESTION: Is it possible to be accurate without being precise??

YES.

In the image below, we see a visualization of “accuracy without precision.’

Accuracy without Precision

The average of the 5 measurements or experiments (the 5 X’s) end up being the bulls-eye!

This is an example of “accuracy without precision.”

— — — — —

ex: An analysis of a compound for its magnesium content gave the following percent compositions:

14.92% Mg , 14.91% Mg , 14.88% Mg , 14.91% Mg

The actual amount of magnesium in the sample is 16.25%.

What conclusions can be made about the accuracy and precision of the results of these 4 experiments to determine percentage composition of magnesium

_________
ANSWER:

We are precise, but not accurate.

Since we are precise, we are not making any human errors (random errors).

But… because our experimental results do NOT agree with the “true value,” an instrumentation error or errors (systematic errors) is/are occurring.

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Accuracy vs. Precision in Chemistry

One last note about systematic error (instrumentation error).

If one’s experimental procedure is “wrong” or “bad,” then we can’t blame the “human” for the errors that are going to result.

In other words, it’s not human error, but instead, a bad experimental procedure would be an example of systematic error.

The same improper procedure gives the same inaccurate results — every time.

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