Accuracy and Precision Measurement Accuracy and Precision

Vocab. to describe good measurements Accuracy – how close the measurement is to the actual value Precision – how well can the measurement be repeated requires several measurements before anything can be said about it

Let’s use a golf anaolgy

Accurate? No Precise? Yes 10

Accurate? Yes Precise? Yes 12

Accurate? No Precise? No 13

Accurate? Yes Precise? We cant say! 18

PRECISE = CONSISTENT(Repeatable) Accuracy vs. Precision ACCURATE = CORRECT PRECISE = CONSISTENT(Repeatable)

Accuracy vs. Precision Good accuracy Good precision Poor accuracy Scientists repeat experiments many times to increase their accuracy. Good accuracy Good precision Poor accuracy Good precision Poor accuracy Poor precision

In terms of measurement Three students measure the width of the classroom to be 10.2 m, 10.3 m and 10.4 m across. Were they precise? Were they accurate?

Measurement Percent Error

Error The difference between an experimental value and an accepted value Error = experimental value – accepted value

Percent Error Tells us the accuracy of an experiment The lower the value the more accurate! Percent error = experimental value – accepted value x 100 accepted value

Practice Example Use Student A’s density data in the table below to calculate the percent error in each trial. Report your answers to two places after the decimal point. The accepted value for the density is 1.59 g/cm3 Density Error Exp - Acc Percent Error Trial 1 1.54 Trial 2 1.60 Trial 3 1.57

Based on the Percent (%) Error Table from Mr Based on the Percent (%) Error Table from Mr. Mayberry’s AP Chem Class, who had the most accurate results: Keith Mary Trevor Bill Percent (%) Error 33.6 % 42.9 % 2.7 % 12.6 %