USING SCIENTIFIC
MEASUREMENTS
Accuracy 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, 15.0cm
Amy:
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:
𝑉𝑎𝑙𝑢𝑒𝑒𝑥𝑝𝑒𝑟𝑖𝑚𝑒𝑛𝑡𝑎𝑙 − 𝑉𝑎𝑙𝑢𝑒𝑎𝑐𝑐𝑒𝑝𝑡𝑒𝑑
𝑃𝑒𝑟𝑐𝑒𝑛𝑡 𝐸𝑟𝑟𝑜𝑟 =
× 100
𝑉𝑎𝑙𝑢𝑒𝑎𝑐𝑐𝑒𝑝𝑡𝑒𝑑
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 (𝑃𝑒𝑟𝑐𝑒𝑛𝑡 𝐸𝑟𝑟𝑜𝑟 =
𝑉𝑎𝑙𝑢𝑒𝑒𝑥𝑝𝑒𝑟𝑖𝑚𝑒𝑛𝑡𝑎𝑙 −𝑉𝑎𝑙𝑢𝑒𝑎𝑐𝑐𝑒𝑝𝑡𝑒𝑑
𝑉𝑎𝑙𝑢𝑒𝑎𝑐𝑐𝑒𝑝𝑡𝑒𝑑
• Substitute (𝑃𝑒𝑟𝑐𝑒𝑛𝑡 𝐸𝑟𝑟𝑜𝑟 =
• Solve (7.7%)
1.40 𝑔/𝑚𝐿−1.30𝑔/𝑚𝐿
1.30 𝑔/𝑚𝐿
× 100 )
× 100 )
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.