1. A CCURACY VS. PRECISION

2. P RECISION Measurements are as only as specific as the instrument being used. Consider a ruler marked in whole inches OR a ruler marked in tenths of inches. This is called the “precision” of the instrument and is indicated by the number of places used in writing the measurement.

3. F OR EXAMPLE …. That ruler marked in whole inches can only be written down to the tenths place. 10.5 1.7 8.3 Matter of fact, since the “tenth” was estimated, anyway, it is called a “guess digit”.

4. H OW ABOUT THE RULER MARKED IN TENTHS ? Well, you could estimate in the hundredths place. 10.58 1.46 0.58 Consider the measurement 11.20 inches using that ruler……why write the “zero”?

5. A CCURACY How close something is to the accepted value In Chemistry – we know the accepted value, or the answer we SHOULD get, for our experiments. Absolute error – exact number we are off from accepted value E value – A value = Absolute error Does that mean much to us? Percent error – percent we are off from accepted value │E value – A value │/ A value * 100

6. E XAMPLE …. John measured the length of the football field and came up with an answer of 98.6 yards. Jane measured a 15.00cm pencil to be 13.80 cm. Who is more precise? Who is more accurate?

7. P RACTICE !! Water has a density of 1.00 g/mL. You calculate water’s density as 0.96 g/mL. What is your percent error? │0.96 - 1.00│ * 100 = 4% 1.00