In DoD, the parameters for a statistical sampling inventory are usually dictated. For example, DLM 4000.25-2 lists the confidence level at 95% with a margin of error of +/- 2.5% for most inventories. That should be enough information to proceed with sampling.
However, I often get asked questions about statistical sampling as it relates to inventory. By far, the most common question that I get asked is how to determine the sample size. There is no shortage of sources, including those in my previous article, that cover this subject and even provide reference tables.
Unfortunately, there is also no shortage of businesses and organizations that take advantage of the situation to try to make money. If you are someone concerned with DoD Financial Improvement Audit Readiness (FIAR), and follow the trail from OMB Circular A-123 to Audit Unit (AU) Section 350, and search for guidance on statistical sampling, you will likely arrive at many for-sale products. The American Institute of Certified Public Accountants publication on audit sampling is one of many examples; at $99 a copy, I cannot recommend it, so please do not waste your money.
Some time ago, after helping someone with a question about sample size, the person asked me how I knew this and whether it was ok for the government to use “my” method. I proceeded to inform that person that statistics is mathematics and, despite what the AICPA wants to sell on their web site, it is not a proprietary method. Therefore, any statistics textbook is a perfectly good reference to validate any statistical method.
So I decided many years ago to develop a spreadsheet meant to illustrate and teach how sample size is calculated. Over the years, I have shared it and people keep asking me for it to this day. If you would like to use it, here is the link: http://www.ncg-consulting.com/files/StatisticalSamplingInventory_v2.xlsx
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