1. Accuracy vs. Precision

2. Accuracy and precision
Accuracy – how close a measurement is to the accepted value
Precision – how close the measured values are to each other
Not accurate, precise
Accurate and precise
Neither accurate nor precise
accurate, not precise

3. Practice – say whether the following are accurate, precise, neither, or both
The width of a piece of paper is supposed to be 8.5 inches. You get the values 8.25 inches, 8.5 inches, 8.5 inches, and 8.5 inches. The average is 8.44 inches.
Suppose a lab refrigerator holds a constant temperature of 38.0 F. A temperature sensor is tested 8 times in the refrigerator. The temperatures from the test yield the temperatures of: 39.4, 39.3, 37.5, 38.3, 39.1, 37.1, 37.8, The average is 38.4

4. More practice – say whether the following are accurate, precise, neither, or both
Suppose a GPS measures your position on earth. You take 3 readings at the same place and they are all close together but 14 miles from your actual position. a b c

5. Even more practice!- say whether the following are accurate, precise, neither, or both
The official time according to an atomic clock is 9:05 pm. Your watch reads 9:06. Your friend’s computer reads 9:05. Your cell phone reads 9:05.
A lab group conducts an experiment to find the density of water and makes three measurements. The results are 0.49 g/cm3, 0.48 g/cm3, and 0.50 g/cm3. A textbook states that the density of water at 20°C is g/cm3.

6. Percent Error Scientists judge accuracy by looking at percent error
If the percent error is 5% or below, then the data is considered accurate

7. Guided # 1 – percent error
The width of a piece of paper is supposed to be 8.5 inches. You measure the value to be 8.8 inches. What is your percent error?
Is this accurate?

8. Standard Deviation
Scientists judge precision by looking at standard deviation
Standard deviation measures how spread out the numbers are

9. Relative or Percent Standard Deviation
It is easier to figure what your standard deviation actually means by using something called relative standard deviation, or percent standard deviation
If the relative standard deviation is 5% or below, then the data is considered precise

10. Guided # 2- standard deviation
A temperature sensor is tested four times in a refrigerator. The temperatures from the test yield the temperatures of: 37.8, 38.3, 37.4 and What is the standard deviation?

11. Guided 2 cont’d

12. A Penny’s Worth Lab
Read and complete the first page of the packet- if not done already.
Collect pennies for your group and get the data using the electronic scales.
Use your notes to complete the standard deviation and the % Error and the rest of the packet.