USING SCIENTIFIC MEASUREMENTSAccuracy and Precision

RHS Science Department

Whachathinkboutit?• Write your definition of the following terms:

• Accuracy• Precision

• Do they mean the same thing or is there a difference?• If there is a difference, what is the difference? (Give an example.)

Accuracy vs. Precision• To most people, the words Accuracy and Precision are

synonymous.• In the field of science, they have quiet distinct meanings.• Accuracy is defined as the closeness of the

measurement to the correct or accepted value of the quantity measured.

• Precision is defined as the closeness of a set of measurements of the same quantity to one another.

Accuracy

Precision

Whachathinkboutit?

BOTH

Whachathinkboutit?

NEITHER

Application of Concept

• Who is more accurate when measuring a book that has a true length of 17.0cm?

Susan: 17.0cm, 16.0cm, 18.0cm, 15.0cmAmy: 15.5cm, 15.0cm, 15.2cm, 15.3cm

Application of Concept

Who is more precise when measuring the same 17.0cm book?

Susan: 17.0cm, 16.0cm, 18.0cm, 15.0cm

Amy: 15.5cm, 15.0cm, 15.2cm, 15.3cm

Error• Due to a variety of factors (eyesight, equipment

limitations, material, etc…) there is a margin of error between our experimental measurement value and the accepted value.

• To calculate the error, we apply the following formula:

Error = Valueexperimental - Valueaccepted

Percentage Error• The amount of error is generally expressed as a

percentage of the accepted value.• To calculate percent error, we apply the following formula:

Practice• A student measures the mass and volume of a substance

and calculates its density as 1.40 g/mL. The correct value of the density is 1.30 g/mL. What is the percentage error of the student’s measurement?

• G.U.E.S.S• Given (measured = 1.40 g/mL; correct = 1.30 g/mL)• Unknown (percent error)• Equation ( )• Substitute ( )• Solve (7.7%)

Interpreting the Results• Percent error can be positive or negative.• If it has a positive value, it means that the experimental or

measured value was greater than the accepted value.• If it has a negative value, it means that the experimental

or measured value was less than the accepted value.