Hi
For sample size for n=2 Stad. Dev would be calculated to 0. So for STD. Dev.you must use sample size greater than 1.
From SAP LIBRARY
- Sample Size
When selecting the sample size, you should take certain quality-relevant conditions into account.
If you want to use the sample size n=1, you should make sure that:
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The control chart does not contain an s track.
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While calculating the action limits, the process dispersion is determined using the total dispersion or the dispersion between samples and not using the internal dispersion.
The reason for this is that the standard deviation of sample size n=1 is always 0.
In the case of SPC inspections with recording of measured values, sample sizes between 4 and 20 are standard; the most frequently used value is n=5.
By choosing a larger sample size, you can use the central specification limit record for quantitative characteristics whose original values do not have a normal distribution. An approximate normal distribution of the mean values is then calculated according to this central specification limit record. Therefore, you can also use Shewhart charts for the mean value even if the population is not normally distributed. The sample size you select should be in accordance with the degree to which the distribution deviates from the standard distribution
For more info.
Maintaining Sampling Procedure for Control Charts - Statistical Process Control (SPC) - SAP Library
Regards
Manish