Accuracy and Precision
Notes
Chemistry 1
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Uncertainty in Measurements
There is no such thing as a perfect
measurement!
• All measurements have a degree of
uncertainty because
– Instruments have flaws
– Measuring always requires estimation on the
last digit
• But we want measurements to be as
accurate and precise as possible
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Accuracy
• The CORRECTNESS of a measurement
• How close a measurement’s value is to
the accepted (actual or true) value
• Can be described for a single
measurement or for the average of a
group of measurements
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• Accuracy is evaluated by
– comparing the experimental value to the accepted
value
• Experimental value is the value that is measured in the lab
• Accepted value is the true or correct value based on reliable
references
– calculating the % error for the measurement
%error = experimental value – accepted value
accepted value
x 100
– The closer the % error is to 0, the better the accuracy
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• Notice that if your % error is negative, that
means your experimental value was too
low
• If your %error is positive, that means your
experimental value was too high
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Calculating % error- examples
A student measured the boiling point of a liquid during a lab
experiment and obtained a value of 99.1◦ C. The
accepted value for the boiling point of the liquid is
102.5 ◦ C. What is the percent error of the student’s
measurement?
Would you classify this student’s measurement as accurate
or inaccurate?
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Lab group #1 measured the mass of a piece of copper and
obtained a value of 10.5 grams. The accepted value for
the mass of this copper is 9.1 grams. Calculate the
%error of this group’s measurement.
Was this lab group’s measurement accurate?
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Precision
• How REPRODUCIBLE a measurement is
• How close in value a group of measurements
are to one another
• Can’t be described for a single measurement;
only for a group of measurements
• Also depends on the smallest divisions (or
increments) on the scale of the measuring
instruments
– The smaller the increments, the more precise the
measurements will be
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• We can evaluate the precision of a group
of measurements by looking at the range
between the smallest and largest
measurement in the group
– The smaller the range, the better the precision
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• Lab group 1 made measurements and
calculated the density of a metal in lab.
For trial #1, their value was 3.45g/cm3. For
trial #2, their value was 3.78g/cm3.
• Lab group #2 had values of 3.12g/cm3 and
3.15g/cm3.
• Which lab group’s measurements were
more precise?
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Examples- Accuracy and Precision
• http://www.carlton.srsd119.ca/chemical/Si
gfigs/accuracy_and_precision.htm
• http://scidiv.bcc.ctc.edu/Physics/Measure&
sigfigs/B-Acc-Prec-Unc.html
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