One of Static Characteristic of Measurement Systems is Random Errors
Revealed by: Avery
On: 05 Jun, 2017
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Random errors are those that remain after the causes of systematic errors have been eliminated...
Random errors are those that remain after the causes of systematic errors have been eliminated. They appear when the same value of the same quantity is measured once and again, using the same instrument and the same method. They have the following properties:
1. Positive and negative random errors with the same absolute value, have the same probability of occurring.
2. Random errors are less probable as the absolute value increases.
3. When the number of measurements increases, the arithmetic mean of random errors in a sample (set of measurements) approaches zero.
4. For a given measurement method, random errors do not exceed a fixed value. Readings exceeding that value should be repeated, and, if necessary, studied separately.
Random errors are also called accidental (or fortuitous) errors, meaning that they may be unavoidable. The absence of changes from one reading to another when measuring the same value of the same quantity several times does not necessarily imply an absence of random errors. It may happen, for example, that the instrument does not have a high enough resolution. That is, that its ability to detect small changes in the measured quantity is rather limited and therefore the user does not perceive them.
The presence of random errors results in an uncertainty of the true value (exact value) of a quantity after measuring it one or several times. The greater that uncertainty evaluated by means of statistical parameters [4, 5), the lesser is the repeatability of the measurement. If in addition there are systematic errors, then the final result will not coincide with the correct one, and the measurement will be inexact.
If the mean value of several readings is taken, then random errors cancel and only systematic errors remain. Since systematic errors are reproducible, they can be determined for some specified measurement conditions, and then the reading can be corrected when measuring under the same conditions. This calculation of the difference between the true value and the measured value is performed during the calibration process under some specified conditions. Furthermore during that process the instrument is usually adjusted to eliminate that error. When making a single measurement under the same conditions, only the random component of error remains.
In practice, however, during the calibration process only systematic err ors for some very specific conditions can be eliminated. Therefore under different measurement conditions some systematic errors even greater than the random ones may be present. These errors are stated by the manufacturer on product data sheets. Usually the range having a given probability of enclosing the true value is stated in the form of difference between the upper and the lower range values, but other ways of specifying that range are also encountered.
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