Precision vs' Accuracy

1
Precision vs. Accuracy

• Whats the difference?

2
Accuracy in Measurement

• Definition-
• A measurement is accurate if it is close to the
  accepted true value of a known quantity.
• Accuracy depends on the observers skill and the
  quality of the instrument.

3
Precision in Measurement

• Definition
• A measurement is precise if it is agrees closely
  with repeated measurements.
• Precision also depends on the observers skill
  and the quality of the instrument.

4
So What?

• Measurements can be accurate without being
  precise. How?
• Replicate measurements can be widely spread apart
  but the average is close to the true value.
• Indicates different types of errors or problems.
• Measurements can be precise without being
  accurate. How?
• Replicate measurements can be close together but
  nowhere near the true value.
• Indicates the same error is repeated.

5
Calculating Percent Error

• How far off are we?

6
Percent Error

• The generic formula for calculating percent error
  is
• observed value true value x 100 percent
  error
• true value

7
Error Problem

• A runner actually covers a certain distance in
  45.4s. You measure 44.8s. Calculate the percent
  error.
• observed value true value x 100
  percent error
• true value
• 44.8-45.4 / 45.4 x 100 -1.32
• Error can be either positive or negative. This
  reflects whether the error was above or below the
  true value.

8
Dimensional Analysis

• Say what?
• A method for solving problems by using conversion
  factors and treating units as algebraic
  variables.
• We will use this method in many different areas
  of chemistry.
• Nonmetric conversions
• Frequency/wavelength/energy problems
• Stoichiometry problems (moles, reaction yields,
  etc.)
• Specific heat/heat of vaporization/crystallization