A CCURACY VS. PRECISION. P RECISION Measurements are as only as specific as the instrument being used. Consider a ruler marked in whole inches OR a ruler.

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Presentation transcript:

A CCURACY VS. PRECISION

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.

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

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

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

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 cm. Who is more precise? Who is more accurate?

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? │ │ * 100 = 4% 1.00