Accuracy and Accuracy and PrecisionPrecision

Remember these definitions?Remember these definitions?

►Accuracy – a description of how close a Accuracy – a description of how close a measurement is to the true value of measurement is to the true value of the quantity measuredthe quantity measured

►Precision – the exactness of a Precision – the exactness of a measurementmeasurement

Accuracy verses PrecisionAccuracy verses Precision

Is it Accurate, Precise, Both or Is it Accurate, Precise, Both or Neither?Neither?

►Known Density = 3.11 g/mLKnown Density = 3.11 g/mL Test Results 3.77, 3.81, 3.76, 3.80Test Results 3.77, 3.81, 3.76, 3.80 Precise, not accuratePrecise, not accurate Test Results 3.01, 3.89, 3.50, 5.99Test Results 3.01, 3.89, 3.50, 5.99 NeitherNeither Test Results 3.04, 3.20, 3.13, 3.07Test Results 3.04, 3.20, 3.13, 3.07 Accurate, not preciseAccurate, not precise Test Results 3.11, 3.12, 3.12, 3.10Test Results 3.11, 3.12, 3.12, 3.10 BothBoth

What are some reasons for What are some reasons for accuracy or precision being off?accuracy or precision being off?

►ErrorErrorHuman ErrorHuman Error►Parallax Error – angle errorParallax Error – angle error►Mathematical ErrorMathematical Error

Instrument ErrorInstrument Error

How do we represent error? How do we represent error?

►Error is the difference between the Error is the difference between the actual (or accepted) value and the actual (or accepted) value and the experimental valueexperimental value

►Percent ErrorPercent ErrorPercent Error = Accepted – ExperimentalPercent Error = Accepted – Experimental

x100x100

AcceptedAccepted

Example ProblemExample Problem

►Working in the laboratory, a student Working in the laboratory, a student finds the density of a piece of pure finds the density of a piece of pure aluminum to be 2.85 g/cm3.  The aluminum to be 2.85 g/cm3.  The accepted value for the density of accepted value for the density of aluminum is 2.699 g/cm3.  What is the aluminum is 2.699 g/cm3.  What is the student's percent error? student's percent error?

Another Example ProblemAnother Example Problem

►A student takes an object with an A student takes an object with an accepted mass of 200.00 grams and accepted mass of 200.00 grams and masses it on his own balance.  He masses it on his own balance.  He records the mass of the object as records the mass of the object as 196.5 g.   What is his percent error? 196.5 g.   What is his percent error?

Which leads us to the labWhich leads us to the lab►Density can be measured in two ways.Density can be measured in two ways.► If it is a liquid:If it is a liquid:

Find the mass of the liquidFind the mass of the liquid Find the volume of the liquidFind the volume of the liquid Divide the mass by the volumeDivide the mass by the volume

► If it is a solidIf it is a solid Find the mass of the solidFind the mass of the solid Submerge the solid in a liquid and record the Submerge the solid in a liquid and record the

difference in the volume of the liquid (final – difference in the volume of the liquid (final – initial)initial)

Divide the mass by the volume differenceDivide the mass by the volume difference