Precision & Accuracy 1

Objectives Distinguish between accuracy and precision. Understand how to make measurements with precision and accuracy. Understand how to identify and quantify causes and effects of uncertainties in data.

Measurements are taken in nearly every lab, but no measurement is perfect. There are two factors: Accuracy Precision

Accuracy The degree of closeness of a measured or calculated quantity to its actual (true) value. 4

Precision The degree to which further measurements or calculations show the same or similar results. Precision describes the limitation of the measuring instrument Significant figures help keep track of imprecision. 5

Accuracy and Precision Precision is the degree of exactness for a measurement. It is a property of the instrument used. The length of the pencil can be estimated to tenths of centimeters. Accuracy is how close the measurement is to the correct value.

Types of Errors made in Lab: Human Method Instrument

Errors in Measurement Instrument error Method error Instrument error is caused by using measurement instruments that are flawed in some way. Instruments generally have stated accuracies such as “accurate to within 1%.” Method error Method error is caused by poor techniques (see picture below).

Let’s Practice You have a known mass of 10. grams. You weigh it on a scale three times. The three readings were: 10.1 g 9.9 g 10.0 g Is the scale accurate, precise, both or neither? Both 9

Let’s Practice Precise You have a known mass of 10. grams. You weigh it on a scale three times. The three readings were 8.1 g 7.9 g 8.2 g Is the scale accurate, precise, both or neither? Precise 10

Let’s Practice Neither You perform an experiment exactly the same three times to obtain a percentage of copper from ore. According to your calculations, you should have extracted 5.0 g of copper. The following is how much copper you actually extracted. 2.0 g 7.9 g 3.2 g Were you accurate, precise, both or neither? Neither 11

Error Analysis We must discuss error in our work. We must minimize error in our work. Percent error 12