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Random Sampling Gives the best estimate
Makes inferences valid<br>
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Random Sampling Methods Determining sample size (n)
First, we need to know the inspection lot size
Consult R87, Table 1 to determine sample size
This allows us to prepare an appropriate random sampling plan<br>
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Random Sampling Table 1 Sampling plans for pre-packages<br>
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Random Sampling Table 1 Sampling plans for pre-packages<br>
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Random Sampling Methods Sampling types
Choosing a random sample not collected from the production line
Choosing a random sample collected from the production line over time<br>
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Random Sampling Methods 1. Choosing a random sample not collected from the production line<br>
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Random Sampling Sampling – not collected from the production line<br>
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Random Sampling Methods of Random Sampling
‘Mechanical’ method
Begin by allocating a reference number to each prepackage in the inspection lot<br>
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Random Sampling Drawing cards from a hat
Record the numbers allocated to the individual prepackages in some physical way
i.e. on cards, slips of paper etc.
After thoroughly mixing, choose as many cards etc. as there are pre-packages needed for the sample.
The corresponding pre-packages of the inspection lot will then make up the sample.<br>
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Random Sampling Random number tables
Any position in the table has a probability of 0.1 of being occupied by any particular digit.
Select a starting point by using a random procedure.<br>
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Random Sampling Random number tables
Stab a page of the table with a pin
the page number
the row number
the column number
to start taking random digits.<br>
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Random Sampling Random number tables
For a lot containing up to 999 pre-packages
001, 002, ….999
ignore any triplet outside the range
similarly for lot sizes between 1000 and 9999 use four digit numbers<br>
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Random Sampling Random number generation using a spread sheet
Spread sheets can generate random numbers by using function “=RANDBETWEEN(low,high)”
low being 0, and
high being the number of pre-packages in the inspection lot<br>
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Random Sampling Random number generation using a calculator
Many calculators can generate random numbers between 0.000 and 0.999 inclusive<br>
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Random Sample Example
Choose a random sample of 50 pre-packages from a lot size of 400.
Allocated each of the 400 packages in the lot their own reference number i.e. 1, 2,….399, 400
A random sequence of individual digits is produced<br>
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Random Sampling 50 randomly generated numbers (0-400)<br>
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Random Sampling Ordered randomly generated numbers (0-400)<br>
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Random Sampling Example
Prepackages numbered 10, 16, 36, 40, 51 and so on are selected and their individual prepackage error is determined<br>
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Random Sampling Methods of Random Sampling
Systematic Sampling
Determine the sample size ‘n’
Divide the lot size by the sample size to determine sampling interval ‘k’
Randomly generate a number to determine your start point
Sample every ‘kth‘ package from the lot<br>
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Random Sampling Methods 2. Choosing a random sample from the production line using time based sampling<br>
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Random Sampling Sampling over time
Reference test of 1 hour’s production.
Random numbers to give random times
To take sample pre-packages after the point of final checks by the packer from the production line<br>
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Random Sampling Sampling over time
Divide the hour into 3600 seconds
choose the random times in the range from 0001 to 3600 for the required sample size<br>
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Random Sampling The following sequence of numbers would produce the corresponding times throughout the hour.
1287 pack being chosen after 21 min 27 sec
1936 “ “ “ “ 32 min 16 sec
1677 “ “ “ “ 27 min 57 sec
0328 “ “ “ “ 5 min 28 sec
etc<br>
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Random Sampling Arrange the times in chronological order and with a stop watch select the pre-packages from the production line
Complete random time exercise now<br>
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Random Sampling Random number generation using a spread sheet
Spread sheets can generate random numbers by using function “=RANDBETWEEN(low,high)”
low being 0, and
high being the number of pre-packages in the inspection lot<br>
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Random Sampling Random number generation using a spread sheet
Spread sheets can generate random numbers by using function “=RANDBETWEEN(0,3600)”<br>
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Random Sampling Random number generation using a spread sheet
Copy and paste numbers into new column, then
Filter random numbers from lowest to highest<br>
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Random Sampling Random number generation using a spread sheet
Insert formula “=C2/86400”<br>
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Random Sampling Random number generation using a spread sheet
Format cells to display in “mm:ss”<br>
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Random Sampling Random number generation using a spread sheet
Expand selection to match required sample size<br>
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Random Sampling Thank you for your attention
Any Questions?<br>