Page 20 UNIT 2- Measurement in Science

Accuracy and Precision These notes go on page 21

Accuracy how close a measurement comes to the actual or true value Think of accuracy like hitting the bullseye on a target.

Precision Can you reproduce the data? Precision is getting the same measurement more than twice. Need more than one measurement. Think of this like hitting the same area of a target more than once.

Can you hit the bull's-eye? Three targets with three arrows each to shoot. Both accurate and precise Precise but not accurate Neither accurate nor precise How do they compare? Can you define accuracy vs. precision?

Accuracy and Precision Accurate and Precise x x x

Accuracy and Precision Not Accurate but they are precise x x x

Accuracy and Precision Accurate but Not Precise x x x

Accuracy and Precision Not Accurate and Not Precise x x x

Precision of Lab Equipment Precision refers to the reproducibility of data The more precise a piece of equipment, the more likely you are to get the same measurement repeatedly.

Precision of Lab Equipment When you look at a piece of equipment, first look at its graduations (markings) and try to find out what the lines mean. The more precise piece of equipment has the smallest increment change between the markings Usually the piece of equipment with the more graduations (lines) is the more precise.

Precision of Lab Equipment 30 22 21 21.0 20.20 20 20 20.0 20.00 Increment = 1 mL Increment = 0.2 mL More Precise

a) or More graduations – your measurements will be more precise

b) or More graduations – your measurements will be more precise

c) or More graduations – your measurements will be more precise

d) or More graduations – your measurements will be more precise

Measuring Precisely: 36.4 mL markings = 1 mL precision = 0.1 mL

Measuring Precisely: 11.65 cm Markings = 0.1 cm Precision = 0.01 cm

d) or More graduations – your measurements will be more precise

Data Sets The data set is accurate if the values are close to the true value (literature value)

Data Sets The data set is precise if all of the values are close to each other.

Accurate or Precise? Several groups of students were testing different balances. They tested each balance several times using a 5.01 g weight.

Accurate or Precise? Mass (g) Yes Yes No Yes Balance Trial #1 #2 #3 4.98 5.01 5.00 B 4.05 3.98 4.00 Yes Yes No Yes

Accurate or Precise? Mass (g) No No Yes No Balance Trial #1 #2 #3 3.57 6.04 4.59 D 5.05 4.90 5.10 No No Yes No