Precision and Accuracy When making measurements, a scientists have to evaluate their data. One way is to look at the precision and accuracy. Precision.

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Presentation transcript:

Precision and Accuracy When making measurements, a scientists have to evaluate their data. One way is to look at the precision and accuracy. Precision is how tightly grouped the data is. Accuracy is how close the overall data is to the target value or theoretical value.

precise but not accurate accurate but not precise both precise and accurate

Ronald, Kevin, and Paul perform an experiment to determine the value of acceleration due to gravity on the Earth (980 cm/s 2 ). The following results were obtained: Ronald ± 12 cm/s 2 Kevin ± 8 cm/s 2 Paul ± 4 cm/s 2. Which results are more precise and which are more accurate?

The known value of the acceleration of gravity is 980 cm/s 2. The closest result to that is Ronald’s at 961 cm/s 2 so his results are the most accurate. While Paul’s results are farther from the known value, his results are the closest to each other at ± 4 cm/s 2 so his results are the most precise.

To assure precision and accuracy, instruments used to make measurements need to be used correctly. This is important because one common source of error comes from the angle at which an instrument is read.

Scales should be read with one’s eye directly above the measure. If the scale is read from an angle, as shown in figure (b), you will get a different, and less accurate, value. The difference in the readings is caused by parallax, which is the apparent shift in the position of an object when it is viewed from different angles (a) (b)

While precision depends on the instrument and the technique used to make the measurement, generally, the device with the finest division on its scale produces the most precise measurement. The precision of an instrument is one-half of the smallest division of the instrument.

A 100-cm long rope was measured with three different scales. The answer obtained with the three scales were: 1 st scale - 99 ± 0.5 cm 2 nd scale - 98 ± 0.25 cm 3 rd scale - 99 ± 1 cm Which scale has the best precision? Precision depends on the instrument. The measurement of the 2 nd scale is the most precise because it is ± 0.25 cm.

The precision of a measurement is one unit of the last significant figure. If you had a box that was 18.2 cm long, the precision of that measurement would be 0.1 cm.