How to Use Random Number Table to Select Workers and Minute for Work Sampling

Working sampling using random number table

The work study may be used to determine the methods and time standards that govern the total performance of the workers. These two factors can be continuously analyzed and improved for the total performance elevation of the operating unit.

The study to improve the current method of doing work is called Method Study

Estimation of standard time taken by a qualified worker to complete a job by a standard method is called Work Measurement. There are three ways of conducting this study –
  1. Time Study
  2. Work Sampling
  3. PMTS (Predetermined motion time study)
Out of all these, this article focuses on work sampling.

Understanding work sampling

Work sampling is a fact-finding tool and has the following two main objectives:
  1. To measure activities and delays while a man is working and the percentage of that he is not working. It means a fair day’s work.
  2. Under certain circumstances, to measure manual tasks that are to establish time stan­dards for an operation. 
It states that the observations recorded on operation are a reliable estimate of the time the operation is in that state, provided, “a sufficient number of observations are taken at random”. Here particular stress should be paid on the words “random” and “sufficient number of observations”.

In this method, the error is likely to occur but the error tends to diminish as the number of samples increases.

Simply put, the initial step is to select a “random” worker and observe them in a “random” observation time slot. This random selection is easy if there are 10 or even 20 people. However, it becomes confusing while selecting random people and time slots for more than 100 people working in a unit.

Random number table

For this purpose, an interesting mathematical tool called “Random Number Table” can be used where all members of the population will have an equal and independent chance of being selected for the sample group.

It is a series of digits (0 to 9) arranged randomly in rows and columns. This chart contains grouped numbers (a group of 4 numbers shown in the chart below) which can extend for more than 4 pages!

Note, that you can select any column or row in the table to select random numbers. 

Figure-1: Random number table

You can download the random number table chart here.

The application of random selection can be easily done using this table. Let us select random time slots and then random workers for sampling in simple examples explained below-

Example-1: Using Random table

A work sampling is to be conducted for 5 workdays, given that the work sampling engineer can work from 9 AM to 5 PM every day; let us choose random minutes to study the workers on day 1.

Total available minutes of study = (8 x 60) = 480 minutes

Minutes can be randomly chosen between 000 and 480 
Figure-2

The table above shows how random minutes between 000 to 480 have been selected for inspection. These minutes, when converted into time, gives us these values -

111th minute --- 11:01 AM
61st minute ----- 11:51 PM
376th minute---- 03:20 PM
320th minute----- 4:16 PM
And so on…. 

Example-2: Using Random table

Out of 12 workers choose 5 workers using a random number table.

First, we assign all possible 2- digit numbers to the 12 workers between 00 and 99
Total 2-digit numbers = 100
It means we can assign (100/12) = 8 numbers to each worker
Remainder 4 values are assigned to the last four workers (I, J, K, L).

As shown in the figure above, the two-digit numbers are read from the top row of the table and those workers are selected to whom these numbers are assigned. 5 random workers are B, C, F, H, and J. 


Note: You can choose any row of the random number table. The first row has been used only for illustration purposes in the examples.

There are many software for industrial engineers now a days which automatically give out random selection of time slots and people for observation. All of those use the above explained algorithm to give out values. Some applications are even android/iOS friendly having integration with Microsoft excel and keep updating real-time readings.

Microsoft Excel has no built-in function to randomly pick cells in excel, but there are ways to generate random numbers using some functions as explained in the article - How to generate random numbers in Excel - RAND and RANDBETWEEN functions at Ablebits.com


References:

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