1. 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).

2. 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).

3. Precision vs Accuracy:
Good precision &
good accuracy
Good accuracy but
poor precision
Good precision but
poor accuracy
Poor precision &
poor accuracy

4. PRECISION VS ACCURACY:
Data collected during a particular Lab:
Density of an Aluminum cube (g/cm3):
Student A Student B Student C
Trial
Trial
Trial
Ave
Accurate
Precise
Actual density of Al = g/cm3 No
Yes No
Yes
Yes No

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

6. 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