Thermo Scientific NITON XL3t 500, User Manual

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Changing Sigma
1-114 NITON XL3 Analyzer User’s Guide Thermo Scientific
Sigma
Sigma is the symbol used for Standard Deviation, a measure of how much a
set of numbers deviates from the mean. For example, each of the three data
sets {0, 0, 14, and 14}, {0, 6, 8, and 14} and {6, 6, 8, 8} has a mean of 7.
Their standard deviations are 7, 5, and 1, respectively. The third set has a
much smaller standard deviation than the other two because its values are all
close to 7. In a loose sense, the standard deviation tells us how far from the
mean the data points tend to be.
The number of standard deviations between the process mean and the
nearest specification limit is given in sigmas. As process standard deviation
goes up, or the mean of the process moves away from the center of the
tolerance, the sigma number goes down, because fewer standard deviations
will then fit between the mean and the nearest specification limit.
Confidence Intervals
Confidence intervals assume that the data are from an approximately
normally distributed population - generally, sums of many independent,
identically distributed random variables tend towards the normal
distribution as a limit. Using this assumption, about 68 % of the values
must be within 1 standard deviation of the mean, about 95 % of the values
must be within two standard deviations, about 99.7 % must lie within 3
standard deviations, and about 99.99% of the values must lie within 4
standard deviations.
The greater the sigma value of the test, the more confident you can be that
the sample is as it appears, but the more difficult and time consuming the
testing must be to verify this. That's why it's important to use the most
appropriate sigma value for the test. By adjusting the sigma value for each
type of test, you can optimize the process for your needs.
Adjusting the Sigma
Values
The sigma values are listed in the column headed " -- ". The default value is
2 sigma. You can change this value by selecting the down arrow next to the
value, which opens up a drop-down menu from which you can select the
desired sigma value by clicking on it.
When you have changed the sigma values to the appropriate number, select
the Save button to save these settings for use. Select the Close button to
return to the previous screen without saving any changes.