Chemistry CP ACCURACY AND PRECISION Objectives Define accuracy

Chemistry CP ACCURACY AND PRECISION

Objectives Define accuracy Define precision Compare accuracy & precision Use significant figures

Accuracy refers to how closely a measurement matches the true or actual values To be accurate only requires the true value (bulls eye) & one measurement (for the arrow to hit the target) Highly accurate data can be costly and difficult to acquire

Precision refers to the reproducibility of the measurement and exactness of description in a number. To decide on precision, you need several measurements (notice multiple arrow holes), and you do not need to know the true value (none of the values are close to the target but all the holes are close together. )

Accuracy & Precision 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”.

Comparing Accuracy & Precision Notice the difference in these pictures. To win the tournament the archers must hit the target the most times. The winner must show accuracy & precision. BAD precision. The 1 st archer has _____ accuracy & ____ BAD The 2 nd archer has _____ accuracy &GOOD ____ precision. GOOD accuracy & GOOD The 3 rd archer has _____ precision. GOOD accuracy & _____ BAD The 4 th archer has _____ precision

Example 1 A sample is known to weigh 3. 182 g. Jane weighed the sample five different times with the resulting data. Which measurement was the most accurate? 3. 200 g 3. 180 g 3. 152 g 3. 189 g

Example 2 Consider the data (in cm) 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, and which student had the most accurate work? Trial 1 Student 14. 8 A Student 14. 7 B Student 14. 4 C Trial 2 Trial 3 Trial 4 Trial 5 14. 7 14. 8 14. 2 14. 6 14. 8 14. 4 14. 5

Solution Most precise: Student C and A have same percision (0. 1 cm difference) Most accurate: Student C (2 were true value, rest within 0. 1 cm) Trial 1 Student 14. 8 A Student 14. 7 B Student 14. 4 C Trial 2 Trial 3 Trial 4 Trial 5 14. 7 14. 8 14. 2 14. 6 14. 8 14. 4 14. 5