Think-Pair-Share 1)What do the terms “accurate” and “precise” mean in every day terms? 2)In the desk measuring activity, using the side with the most number of increments (compared to using the side with no increments) increased the… a. accuracy of the measurement b. precision of the measurement

Precision and Accuracy Careful measurements are critical in scientific investigations because… No measurement can be absolutely certain All measurements include a degree of uncertainty 6.6 mL ± 0.1 mL6

Causes of uncertainty 1)Skill and care of person making the measurement 2)Limitations of the measuring instrument

ACCURACY How close a measurement is to the theoretical, or true value. The theoretical value of the density of water is 1g/mL. How accurate were your measurements from Friday’s lab?

PRECISION a)Reproducibility or consistency. b) The degree of exactness or refinement of a measurement(an instrument with smaller increments will allow for a more precise measurement)

EXAMPLE: Data Set 1 Data Set mL 2.7 mL 3.0 mL 3.4 mL 3.1 mL 2.6 mL High Low Precision Precision Changed from notes

How would you describe the following diagrams in terms of accuracy and precision? Click on the numbers

How would you describe the following diagrams in terms of accuracy and precision? Click on the numbers

Accuracy and Precision Scenario A chemist is asked to determine the concentration of a chemical dissolved in a solution. The chemist performs the experiment three times for good measure, and the concentration determined to be 1.74 M, (moles/liter), 1.73 M and 1.75 M. The average of these numbers is 1.74 M. This result is extremely precise, but suppose the chemist is not a very good chemist and made the same mistake in all three experiments: the true concentration of the chemical in solution is 2.32 M. Even though this experiment was done three times, and the concentration was determined very precisely, it is not an accurate result. Now let's say that another chemist performs three more experiments to determine the concentration of the same chemical in solution, and finds the following values: 2.87, 1.48 and 2.61 M. When averaged, these values accurately give 2.32 M, but the experiments were not precise. In fact, it may have been lucky that they averaged out perfectly.

Percent Error A measure of accuracy; how far the experimental or measured value is from the theoretical value. Experimental Value - Theoretical Value

Percent Error Positive Error: experimental value is too high compared to the theoretical value Ex: True value of density of water is 1.0 g/mL. You and your lab partner calculate 1.3 g/mL as density of water. Negative Error: experimental value is too low compared to the theoretical value Ex: True value of density of water is 1.0 g/mL. You and your lab partner calculate 0.9 g/mL as density of water.

Now you try! Using data set 1 and 2 above, calculate the percent error for each, if the theoretical value is 3.0 mL.

% error = actual-theoretical x 100 theoretical Theoretical value = 3.0 mL Set 1 average = 3.07 mL % error = 3.07 – 3.0 x100 = (+) 2.33% 3.0 Set 2 average = 2.9 mL (includes 2.7 mL value) % error = 2.9 – 3.0 x100 = (-) 3.33% 3.0