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
Test Results 3.01, 3.89, 3.50, 5.99
Test Results 3.04, 3.20, 3.13, 3.07
Test Results 3.11, 3.12, 3.12, 3.10
1. Precise, not accurate
2. neither
3. Accurate not precise
4. Both

5. What are some reasons for accuracy or precision being off?
Error
Human 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 g/cm3.  What is the student's percent error?

8. Another Example Problem
A student takes an object with an accepted mass of grams and masses it on his own balance.  He records the mass of the object as g.   What is his percent error?

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