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Accuracy vs. Precision

Accuracy vs. Precision. What’s the Diff?. Accuracy. Accuracy refers to how closely a measurement matches true or actual values To be accurate only requires the true value (the bulls eye ) and one measurement (one arrow to hit the target ). Precision.

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Accuracy vs. Precision

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  1. Accuracy vs. Precision What’s the Diff?

  2. Accuracy • Accuracy refers to how closely a measurement matches true or actual values • To be accurate only requires the true value (the bulls eye) and one measurement (one arrow to hit the target).

  3. Precision • Precision refers to the reproducibilityof a measurement. • Requires several measurements • (notice multiple arrow holes) • Has nothing to do with the true value • (none of the values are close to the target but all the holes are close together).

  4. Does this picture represent accuracy or precision? • Accuracy • Precision • Both accurate and precise

  5. Does this picture represent accuracy or precision? • Accuracy • Precision • Both accurate and precise

  6. Accurate and Precise • In order to be accurate and precise, one must pay close attention to detail to receive the same results every time as well as “hit the target”.

  7. A sample is known to weigh 3.182g. Jane weighed the sample five different times with the resulting data: 3.200g, 3.180g, 3.152g, 3.126g, & 3.189g. Which measurement was the most accurate? • 3.200g • 3.180g • 3.152g • 3.126g • 3.189g

  8. Problem #1 Answer • Answer: The most accurate measurement would be 3.180 g, because it is closest to the actual weight of the sample.

  9. Consider the data (in cm) obtained for the length of an object as measured by three students. The length is known to be 14.5 cm. Which student had the most precise work? • Student A • Student B • Student C

  10. Consider the data (in cm) obtained for the length of an object as measured by three students. The length is known to be 14.5 cm. Which student had the most accurate work? • Student A • Student B • Student C

  11. Problem #2 Answer • Answer: Student A had the most precise work since there is only 0.1 cm between the highest and lowest values, and Student C had the most accurate work since two of the measurements were exactly at the true value and the other three measurements were within 0.1 cm of the actual value.

  12. Limits of Measurement (Fig 20, p. 25) Accuracy: A description of how close a measurement is to the true value of the quantity measured What is meant by this statement? “You can only be as accurate as your instrument.”

  13. How precise can you be? • Can we all agree that it is at least 8 cm? • Will we all have the same number next? 8.3 cm??? • We can only estimate one place beyond what we are sure of…so we will say we are precise to +/- 0.1 cm • In other words…I am confident that you will all measure this within .1 cm of 8.3 cm. szdfasdadfadfasdf szdfasdadfadfasdf

  14. How precise can you be now? • I can now be confident that we will all say this is 8.2. • Now what would you estimate to? • Can I confidently say that we will all measure this as 8.22 +/- .01 cm. • Is there another way I could write that? • Yes! 8.22 cm +/- .1 mm

  15. Limits of Measurement (Fig 20, p. 25) Precision of a calculated answer is limited by the least precise measurement used in the calculation (*determined by significant figures) Example: A ruler with millimeters would give you a more precise measurement than a ruler with just centimeters. When reading an instrument…You may only estimate ONE place beyond the smallest unit actually measured!

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