Accuracy vs. Precision

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

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, 39.0. The average is 38.4

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

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 0.99823 g/cm3.

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

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?

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

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

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 38.5. What is the standard deviation?

Guided 2 cont’d

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.