Accuracy vs. Precision K. Culbertson 2010.

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Presentation transcript:

Accuracy vs. Precision K. Culbertson 2010

1 What does the bulls-eye below show? Accuracy Precision Both Neither

2 What does the bulls-eye below show? Accuracy Precision Both Neither

3 What does the bulls-eye below show? Accuracy Precision Both Neither

4 What does the bulls-eye below show? Accuracy Precision Both Neither

Accuracy Definition The closeness of a measurement to its actual value

Reproducibility Repeatability Precision Definition The degree to which measurements show the same or similar results Reproducibility Repeatability

5 Which team was most precise for the two trials? g/cm3 Team 1 Team 2 Team 3 Team 4 Team 5 Team 6 Trial 1 2.75 2.80 2.70 2.71 2.68 Trial 2 2.76 2.73 2.69 2.65 TEAM 6 = SAME DATA IN BOTH TRIALS The density of aluminum is known to be 2.70 g/cm3

6 Which team was most accurate for the two trials? g/cm3 Team 1 Team 2 Team 3 Team 4 Team 5 Team 6 Trial 1 2.75 2.80 2.70 2.71 2.68 Trial 2 2.76 2.73 2.69 2.65 TEAM 4 = AVG 2.70 The density of aluminum is known to be 2.70 g/cm3

Percent Error Calculation Measured value – accepted value x 100 accepted value Accepted Measured True Observed Actual Your Result Theoretical Experimental

7 A. 2% E. -2% B. 20% F. -20% C. 25% G. -25% D. 80% H. -80% The actual value is 10.0, but the student measures 8.0. What is the percent error? A. 2% E. -2% B. 20% F. -20% C. 25% G. -25% D. 80% H. -80% F

8 A student finds the density of carbon monoxide to be 1.21 g/L at STP. If the accepted value for the density of carbon monoxide at STP is 1.25 g/L, what is the percent error? 0.04% E. -0.04% 3.2% F. -3.2% 3.31% G. -3.31% D. 96.8% H. -96.8% F

9 A student finds the specific heat of water to be 4.29 J/g x Co. Assuming the true value is 4.18 J/g x Co, calculate his percent error. 0.026% E. -0.026% 2.56% F. -2.56% 2.63% G. -2.63% 97.4% H. -97.4% C

How Can We Increase Precision in the Lab? Be honest when reporting a measurement, so that it does not appear to be more accurate than the equipment used to make the measurement allows. We can achieve this by controlling the number of digits, or significant figures, used to report the measurement.