Activity 3.8 Precision Accuracy Measurement
Introduction
This concept of random and systematic errors is related to the precision and accuracy of measurements. Precision characterizes the system's probability of providing the same result every time a sample is measured (related to random error). Accuracy characterizes the system's ability to provide a mean close to the true value when a sample is measured many times (related to systematic error). We can determine the precision of a measurement instrument by making repeated measurements of the same sample and calculating the standard deviation of those measurements. However, we will not be able to correct any single measurement due to a low precision instrument. Simply stated, the effects of random uncertainties can be reduced by repeated measurement, but it is not possible to correct for random errors.
We can determine the accuracy of a measurement instrument by comparing the experimental mean of a large number of measurements of a sample for which we know the "true value" of the characteristic of the sample. A sample for which we know the "true value" would be our calibration standard. We may also need to characterize the accuracy of the measurement instrument by observing historical trends in the distribution of measured values for the calibration standard (this allows for determining the systematic error expected from environmental effects, etc.). The effects of systematic uncertainties cannot be reduced by repeated measurements. The cause of systematic errors may be known or unknown. If both the cause and the value of a systematic error are known, it can be corrected for by "subtracting" the known deviation. However, there will still remain a systematic uncertainty component associated with this correction.
This concept of random and systematic errors is related to the precision and accuracy of measurements. Precision characterizes the system's probability of providing the same result every time a sample is measured (related to random error). Accuracy characterizes the system's ability to provide a mean close to the true value when a sample is measured many times (related to systematic error). We can determine the precision of a measurement instrument by making repeated measurements of the same sample and calculating the standard deviation of those measurements. However, we will not be able to correct any single measurement due to a low precision instrument. Simply stated, the effects of random uncertainties can be reduced by repeated measurement, but it is not possible to correct for random errors.
We can determine the accuracy of a measurement instrument by comparing the experimental mean of a large number of measurements of a sample for which we know the "true value" of the characteristic of the sample. A sample for which we know the "true value" would be our calibration standard. We may also need to characterize the accuracy of the measurement instrument by observing historical trends in the distribution of measured values for the calibration standard (this allows for determining the systematic error expected from environmental effects, etc.). The effects of systematic uncertainties cannot be reduced by repeated measurements. The cause of systematic errors may be known or unknown. If both the cause and the value of a systematic error are known, it can be corrected for by "subtracting" the known deviation. However, there will still remain a systematic uncertainty component associated with this correction.
Conclusion
1. Why is it important to know the accuracy and precision of a measuring device? Knowing the accuracy and precision of a measuring device is important so you can get the best collection of data and to prevent your numbers from being off in any way.
2. Do you think that the dial caliper manufacturer’s claim that the “accuracy” of the instrument is ±.001 is appropriate? Why or why not? Yes, that claim is appropriate because the instrument provides measurements up to the thousandths place.
3. Do you think that either of the dial calipers needs to be adjusted in order to accurately display measurements? Explain. Dial Calipers do need to be calibrated and adjusted to display accurate measurements. If when the dial caliper is closed and the dial is not set to "0," the measurements that a person collects will not be accurate.
1. Why is it important to know the accuracy and precision of a measuring device? Knowing the accuracy and precision of a measuring device is important so you can get the best collection of data and to prevent your numbers from being off in any way.
2. Do you think that the dial caliper manufacturer’s claim that the “accuracy” of the instrument is ±.001 is appropriate? Why or why not? Yes, that claim is appropriate because the instrument provides measurements up to the thousandths place.
3. Do you think that either of the dial calipers needs to be adjusted in order to accurately display measurements? Explain. Dial Calipers do need to be calibrated and adjusted to display accurate measurements. If when the dial caliper is closed and the dial is not set to "0," the measurements that a person collects will not be accurate.