1. Accuracy & Precision

2. Accuracy
Refers to how close a measured value is to the accepted or true value.

3. Precision
Refers to how close a series of measurements are to each other.
Precise measurements show little variation over a series of trials. That means, there’s not much scatter in the data.
Precise measurements are not necessarily accurate!

4. a) c)       b)  d)      Not Accurate Accurate and Precise!
Not Precise
Accurate and Precise!    b)  d) 
Precise but not Accurate
Accurate but
Not Precise    

5. Now let’s look at some data.
These students were asked to determine the density of sucrose. Sucrose has a density of 1.59 g/cm3.
Student A
(g/cm3)
Student B
Student C
Trial 1
1.54
1.40
1.70
Trial 2
1.60
1.68
1.69
Trial 3
1.57
1.45
1.71
Avg.
1.51
Range
0.06
0.28
0.02

6. Student A Student C Which student was most accurate?
Which student was most precise?
Student A
Average value closest to accepted value.
Student C
Smallest range (high minus low value).

7. Let’s summarize: Accuracy refers to …
Precision refers to …
how close your data is to the true or accepted value.
how close your data points are to each other. Data that has a lot of scatter is not precise.