Accuracy vs.

Precision

What’s the Diff?

Accuracy

• Accuracy refers to how closely a measurement matches true or actual values– To be accurate only requires the true value (the bulls

eye) and one measurement (one arrow to hit the target).

Precision• Precision refers to the reproducibility of a

measurement.• Requires several

measurements • (notice multiple arrow

holes)

• Has nothing to do with the true value • (none of the values are

close to the target but all the holes are close together).

Does this picture represent accuracy or precision?

1. Accuracy

2. Precision

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Does this picture represent accuracy or precision?

1. Accuracy

2. Precision

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Accurate and Precise• In order to be accurate and precise, one

must pay close attention to detail to receive the same results every time as well as “hit the target”.

A sample is known to weigh 3.182g. Jane weighed the sample five different times with the resulting data: 3.200g, 3.180g, 3.152g,

3.126g, & 3.189g. Which measurement was the most accurate?

20%

20%

20%

20%

20% 1. 3.200g

2. 3.180g

3. 3.152g

4. 3.126g

5. 3.189g

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Problem #1 Answer

• Answer: The most accurate measurement would be 3.180 g, because it is closest to the actual weight of the sample.

Consider the data (in cm) obtained for the length of an object as measured by three students. The

length is known to be 14.5 cm. Which student had the most precise work?

33%

33%

33% 1. Student A

2. Student B

3. Student C

Consider the data (in cm) obtained for the length of an object as measured by three students. The

length is known to be 14.5 cm. Which student had the most accurate work?

33%

33%

33% 1. Student A

2. Student B

3. Student C

Problem #2 Answer

• Answer: Student A had the most precise work

since there is only 0.1 cm between the highest

and lowest values, and Student C had the most

accurate work since two of the measurements

were exactly at the true value and the other three

measurements were within 0.1 cm of the actual

value.

In order to win the archery tournament, the archer must hit the target the most times.The winner must be accurate and precise.The first archer is just precise. He or she hit the same area of the target every timeThe second archer was accurate because he hit the target once.However, the third archer was accurate and precise because he or she hit the target every time. Therefore, he or she wins!

Poor accuracy

Good precision

Good accuracy

Poor precision

Good accuracy

Good precision

Precision and Significant Digits

• The last digit of a measurement is an estimate, but is still significant

Precision of an Instrument

• Reflects the number of sig. digits in a reading

• All measurements made with the same instrument have same precision

Precision of an Instrument

Reading is btwn. 5.3 and 5.4 cm. What is the precision of this ruler? Hundredths place

Significant Figures

• Used to report all precisely known numbers plus one estimated digit