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 show precision).

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 but poor precision Good precision but poor 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 C Trial 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 Accurate Precise Actual density of Al = 2.699 g/cm3 No Yes No Yes Yes No

III- Percent error: Is the ratio of an error to an accepted value. Percent error = Error = Experimental value – Accepted value. x 100

Example: Calculate the percent error for the students A & B. 2.672 g/cm3 1.540 g/cm3 a)% error = b) % error =   experimental good accuracy X 100 = X 100 = 1.000 % X 100 = 42.94 % X 100 = poor accuracy