I-Precision: Refers to how close a series of measurements are to one another. Examples: 2.34 cm; 2.33 cm; 2.35 cm 2.25 cm; 4.33 cm; 6.82 cm

(show precision).(don’t showprecision).

II- Accuracy: Refers to how close a measured value is to an accepted value (the real one). Example: If a density of an aluminum cube is 2.7 g/cm3 and you get the following measurements in the Lab: a) d = 2.67 g/cm3 (Experimental)

(You have been accurate). b) d = 2.39 g/cm3 (Experimental)

(You measurement didn’t show accuracy).

Precision vs Accuracy:

Good precision &good accuracy

Good accuracy butpoor precision

Good precision butpoor accuracy

Poor precision &poor accuracy

PRECISION VS ACCURACY:Data collected during a particular Lab:Density of an Aluminum cube (g/cm3): Student A Student B Student CTrial 1 2.632 1.532 2.132 Trial 2 2.725 1.525 1.525 Trial 3 2.660 1.562 3.262 Ave. 2.672 1.540 2.306 Student A Student B Student CAccurate Precise

Actual density of Al = 2.699 g/cm3

Yes No Yes

No Yes

No

III- Percent error: Is the ratio of an error to an accepted value. Percent error =

Error = Experimental value – Accepted value.

valueAccepted

error

...x 100

Example: Calculate the percent error for the students A & B.a)2.672 g/cm3

b)1.540 g/cm3

a)% error =

b) % error =

experimental

699.2

699.2672.2 X 100 1.000 % =

699.2

027.0X 100 =

699.2

699.2540.1 X 100 =

699.2

159.1X 100 =

42.94 %

good accuracy

pooraccuracy