Accuracy and Precision

1. Accuracy and Precision

2. Remember these definitions? • Accuracy – a description of how close a measurement is to the true value of the quantity measured • Precision – the exactness of a measurement

3. Accuracy verses Precision

4. Is it Accurate, Precise, Both or Neither? • Known Density = 3.11 g/mL • Test Results 3.77, 3.81, 3.76, 3.80 • Precise, not accurate • Test Results 3.01, 3.89, 3.50, 5.99 • Neither • Test Results 3.04, 3.20, 3.13, 3.07 • Accurate, not precise • Test Results 3.11, 3.12, 3.12, 3.10 • Both

5. What are some reasons for accuracy or precision being off? • Error • Human Error • Parallax Error – angle error • Mathematical Error • Instrument Error

6. How do we represent error? • Error is the difference between the actual (or accepted) value and the experimental value • Percent Error Percent Error = Accepted – Experimental x100 Accepted

7. Example Problem • Working in the laboratory, a student finds the density of a piece of pure aluminum to be 2.85 g/cm3.  The accepted value for the density of aluminum is 2.699 g/cm3.  What is the student's percent error? % error = |2.699 g/cm3 – 2.85 g/cm3| x 100 2.699 g/cm3 = 5.60 %

8. Another Example Problem • A student takes an object with an accepted mass of 200.00 grams and masses it on his own balance.  He records the mass of the object as 196.5 g.   What is his percent error? % error = |200.00 g – 196.5 g| x 100 200.00 g = 1.75 %

9. Which leads us to the lab • Density can be measured in two ways. • If it is a liquid: • Find the mass of the liquid • Find the volume of the liquid • Divide the mass by the volume • If it is a solid • Find the mass of the solid • Submerge the solid in a liquid and record the difference in the volume of the liquid (final – initial) • Divide the mass by the volume difference