Cliffs AP Chemistry, 3rd Edition

Precision

Often an actual or accepted value is not known. If this is the case, the accuracy of the measure-

ment cannot be reported since one does not know how close or far one is from the actual value.

Instead, experiments are repeated several times and a measurement of how close together the

values lie (precision) is done. It is the goal that experiments that give reproducible results will

also give accurate results.

Absolute deviation =|Measured −Mean|

Average Deviation or Average Difference =Average of all the absolute deviations.

Percent Deviation Mean

Average Deviation

= #100%

Example: Given three masses of the same object: 1.51 g, 1.63 g, 1.48 g

Mean or Average ....

3

=151 163 148++= 154

Absolute Deviation of each value from mean:

|1.51 −1.54| =0.03

|1.63 −1.54| =0.09

|1.48 −1.54| =0.06

Average Deviation ....

3

=003 009 006++= 006

Relative Deviation for Relative Difference Mean %

Average Deviation

= # 100

. %.%
154
==^006 # 100 3 9

This says that the three measurements are within 3.9% of the average (and hopefully) true

value of the object.

Rounding Off Numbers

318.04 =318.0 (the 4 is smaller than 5)

318.06 =318.1 (the 6 is greater than 5)

318.05 =318.0 (the 0 before the 5 is an even number)

318.15 =318.2 (the 1 before the 5 is an odd number)

Part I: Introduction

8684-X Ch01.F 2/14/01 2:49 PM Page 18