Accuracy, Precision, and a whole lot of Uncertainty

Accuracy & Precision

What is Accuracy?

• Accuracy is how close a measured value is to the actual (real) value.

What is Precision?

• Precision is how close the measured values are to each other.

For example:

Big black circle: Low Precision

Small Black circle: High Precision

Farther away from Bull's Eye: Low Accuracy

Close to Bull's Eye: High Accuracy



High Accuracy, High Precison



High Accuracy, Low Precision



Low Accuracy, High Precision





Measurement and Uncertainty

No measurement is exact. It is only a best estimate, which has some degree of uncertainty. We only get an exact number when we count a set of objects.

• e.g. there are 23 books on the bookshelf

Absolute Uncertainty

Uncertainty is expressed in the units of measurement, not as a ratio.

To determine absolute uncertainty:

Method 1:

• Discard unreasonable data. Calculate the average of at least 3 measurements. The absolute uncertainty is the largest difference between the average of the highest and the lowest reasonable measurement.

Method 2:

• Determine the uncertainty of each instrument used. Make sure the measurement(s) is as precise as possible. Estimate to 0.1 of the smallest segment on the instrument scale.

e.g. if the smallest measurement on a ruler is in centimetres (cm), measure to 0.1cm, or 1 millimetre (mm)

Relative

Uncertainty  = absolute uncertainty / estimated measurement

the ratio can be expressed:

                                        (1) in percent (%)

                                        (2) using significant figures

Here are some videos to help clearify anything you do not understand.

Accuracy and Precision



Measurement and Uncertainty



FUN FACT OF THE DAY:
DID YOU KNOW... each cell in your body has more molecules that there are stars in the Milky Way Galaxy?

By: JZ