Work sampling is a work measurement methodology that estimates the proportion of time an employee utilizes in performing assigned job tasks. The methodology uses random observations of actual worker activity and is dependent on the laws of probability. Since it does not require a formalized time study procedure conducted by qualified stopwatch analysts, it is less costly. The methodology requires that the manager simply determine whether an employee is actually working or is idle during any particular observation.
After all the observations have been completed, the percentage of working observations is computed from the total observations. The greater the number of observations, the more accurate the technique is.
How is work sampling used?
Work sampling is used for the following:
1. Ratio delay studies. Worker allowances are determined by calculating the percentage of time an employee spends on unavoidable delays.
2. Percent utilization of equipment. The technique is used to determine the actual utilization of machinery and other equipment.
3. Determining labor standards. The technique is useful in determining work standards for various tasks by rating the employee`s performance.
4. Evaluating employee performance. A performance standard can be calculated utilizing the work sampling procedure and resulting standards.
How is work sampling performed?
1. Sample observations. Several sample observations are performed to act as the basis for developing a correct sample size based on the problem`s parameters.
2. Compute the actual sample size. The sample size is dependent on the desired level of statistical confidence and accuracy. Normally, the acceptable level of confidence is 95% with an accuracy level of ± 5%. The following formula determines the actual sample size necessary for a work sampling procedure:
N = (Z2 (1 - p)) / a2p
p = estimate of time utilized in an activity
1-p = estimate of time not utilized in an activity
a = accuracy level fraction
Z = confidence level (Z = 1 for 68.3% confidence level, Z= 2 for 95.5% confidence level, and Z= 3 for 99.7% confidence level)
A higher confidence level and a reduced accuracy level fraction will increase the required sample size. The 95% confidence level and ± 5% accuracy level establish that in 95 cases out of 100 the sampling activity will be accurate within ± 5% of the proportion of time utilized in an activity (p).
3. Prepare a random schedule of employee observations.
4. Observe and rate the employee`s work performance.
5. Total the number of units produced and calculate the normal time per unit.
6. Compute the standard time per unit.
The supervisor of a large production organization wants to determine what the idle time is with a confidence level of 95.5% and an accuracy level of 5%. After performing a random sample of 75 observations, it is determined there is 20% idleness. Analyze the percentage of operational idleness.
The required sample size is determined by using the following formula:
Z = 95.5% confidence level = 2
N = (Z2 (1 - p) / a2p = (4 (1- .80)) / (0.0025(0.80)) = 400Additional observations needed for sample = 400 - 75 = 325
For establishing labor standards, work samples are used in a similar manner to time studies. However, work samples, are more appropriate for operations having long production cycles, group service or production operations, and work using indirect labor. A determination is made as to whether the employee is busy or idle during the observation, a ratio is given to the employee, and the units produced are totaled in order to produce an average. Using this data, the normal time and standard time can be determined:
Normal time = ((Total study time) x (working timepercent) x (performance rating)) / numbers of units produced
The standard time is the normal time plus allowance time:
standard time = normal time + allowance time
= normal time / 1 - allowance time
A work sample study of a production operator was conducted over 60 hours (3,600 minutes) and disclosed the following data:
|Number of pieces produced||580|
|Total number of observations||800|
| Total number of observations working
|Average performance rating||95%|
The total allowance given by the company for this operation is 15%. What is the standard time for each operation?
Normal time = ((Total study time) x (working time percent) x (performance rating)) / numbers of units produced
= ((3600 min.)(0.8125)(.95)) / 580 = 4.8 minutes / unit of production
Standard time = normal time / 1 - allowance time
= 4.8 / (1- .15) = 5.65 minutes / unit of production