Training and consultancy for testing laboratories.

Sampling 8

The open source R programing language is a free software environment for statistical computing and graphics, and is easy to master. The official website is https://www.r-project.org/ . It can run on a wide variety of UNIX platforms, Windows and MacOS.

On September 24, 2016, this blog site published an article on how to use R to generate random numbers (https://consultglp.com/2016/09/24/how-to-use-r-to-generate-random-numbers/) .   In light of the newly revised ISO/IEC 17025 accreditation standards embracing sampling as another important criterion for technical competence assessment, the random number function of R becomes very handy for cargo surveyors and samplers to prepare their sampling plan on cargo shipment.

We can use the random number function of R to create a random number table to suit the needs in randomly selecting samples for laboratory quality analysis.

For example, there is a shipment of 1000 bags of coffee beans in a warehouse to be surveyed prior to be dispatched to port. The buyer requires a 5% sampling for laboratory quality testing.  That means some 50 bags have to be random selected before composite a portion of each bag into a suitable sized test sample through a quartering sub-sampling process.

The sampling plan, therefore, can be the following process:

1.  Label each bag with a sequential number

2.  Create 50 numbers in a random number table with the R command language:

> RandSampling=sample(500,50)

> dim(RandSampling)=c(10,5)

> RandSampling

[,1]   [,2]   [,3]   [,4]   [,5]

[1,]  154  424   84  486   82

[2,]   78  214  275  498  388

[3,]   93  104  478  148  258

[4,]  229  283   96  479  489

[5,]  487  211  216   59  263

[6,]   94  450   47  201  105

[7,]  330  121  130  276   56

[8,]   11  415  303  240  407

[9,]  427   60   71  142  409

[10,]  101  238  228  441  355

>

3.  Sample a portion (say, 500g) of the coffee beans from the bags with these selected numbers into a large sampling bag.

4.  Conduct a sample quartering process on site to reduce the test sample size to about 2.5 kg before sending to the laboratory for analysis.

 

 

 

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