Time Studies
The classical stopwatch study, or time study, originally proposed by Federic W. Taylor in 1881, is still the most widely used time study method. The time study procedure involves the timing of a sample of worker’s performance and using it to set a standard. A trained and experienced person can establish a standard by following these eight steps (Heizer et al., 2000, p. 409-412).
1. Define the task to be studied (after methods analysis has been conducted).
2. Divide the task into precise elements (parts of a task that often takes no more than a few seconds).
3. Decide how many times to measure the task (the number of cycles of samples needed).
4. Record elemental times and rating of performance.
5. Compute the average observed cycle time. The average observed cycle time is the arithmetic mean of the times for each element measured, adjusted for unusual influence for each element :
6. Determine performance rating and then compute the normal time for each element.
Normal Time = (average observed cycle time) x (performance rating factor).
7. Add the normal times for each element to develop a total normal time for each task.
8. Compute the standard time. This adjustment to the total normal time provides allowances such as personal needs, unavoidable work delays and worker fatigue.
Personal time allowances are often established in the range of 4% to 7% of total time, depending upon nearness to rest rooms, water fountains, and other facilities. Delay allowances are often set as a result of the actual studies of the delay that occurs. Fatigue allowances are based on our growing knowledge of human energy expenditure under various physical and environmental conditions. The major two disadvantages of this method are; first they require a trained staff of analysts and secondly the labor standards cannot be set before tasks are actually performed.
Predetermined Time Standards
Predetermined time standards divide manual work into small basic elements that already have established times (based on very large samples of workers). To estimate the time for a particular task, the time factors for each basic element of that task are added together. Developing a comprehensive system of predetermined time standards would be prohibitively expensive for any given firm. Consequently, a number of systems are commercially available. The most common predetermined time standard is methods time measurement (MTM), which is the product of the MTM association (Heizer et al., 2000 p. 415-416).
Predetermined time standards are an outgrowth of basic motions called therblings. The term "therblig" was coined by Frank Gilbreth. Therbligs include such activities as select, grasp, position, assemble, reach, hold, rest and inspect. These activities are stated in terms of time measurement units (TMUs), which are each equal to only 0.00001 hour or 0.0006 minutes. MTM values for various therbligs are specified with the help of detailed tables.
Predetermined time standards have several advantages over direct time studies. First, they may be established in laboratory environment, where the procedure will not upset actual production activities. Second, because the standard can be set before a task is actually performed, it can be used for planning. Third, no performance ratings are necessary. Fourth, unions tend to accept this method as fair means of setting standards. Finally, predetermined time standards are particularly effective in firms that do substantial numbers of studies of similar tasks.
Work Sampling
It is an estimate of the percentage of time that a worker spends on particular work by using random sampling of various workers. This can be conducted by the following procedures (Heizer et al., 2000, p. 416-418).
1. Take a preliminary sample to obtain an estimate of the parameter value (such as percent of time worker is busy).
2. Compute the sample size required.
3. Prepare a schedule for observing the worker at appropriate times. The concept of random numbers is used to provide for random observation. For example, let’s say we draw the following 5 random numbers from a table: 07, 12, 22, 25, and 49. These can then be used to create and observation schedules of 9:07 AM,
9:12, 9:22, 9:25, and 9:49 AM.
4. Observe and record worker activities.
5. Determine how workers spend their time (usually as percentage).
To determine the number of observation required, management must decide upon the desired confidence level and accuracy. First, however, the analyst must select a preliminary value for the parameter under study. The choice is usually based on small sample of perhaps 50 observations. The following formula then gives the sample size for a desired confidence and accuracy.
n = Z2 ∗ p &1 − p)/h2
Where, n = required sample size
z = standard normal deviate for the desired confidence level (z = 1 for 68% confidence, z = 2 for 95.45% confidence, and z = 3 for 99.73% confidence level)
p = estimated value of sample proportion (of time worker is observed busy or idle)
h = acceptable error level, in percent
Work sampling offers several advantages over time study methods. First, because a single observer can observe several workers simultaneously, it is less expensive. Second, observers usually do not require much training and no timing devices are needed. Third, the study can be temporarily delayed at any time with little impact on the results. Fourth, because work sampling uses instantaneous observations over a long period, the worker has little chance of affecting the study outcome. Fifth, the procedure is less intrusive and therefore less likely to generate objections. The disadvantages of work sampling are:
1. It does not divide work elements as completely as time studies.
2. It can yield biased or incorrect results if the observer does not follow random routes of travel and observation.
3. Being less intrusive, it tends to be less accurate; this is particularly true when cycle times are short.
Method Study
Method study focuses on how a task can (should) be accomplished. Whether controlling a machine or making or assembling components, how a task is done makes a difference in performance, safety, and quality. Using knowledge from ergonomics and methods analysis, methods engineers are charged with ensuring quality and quantity standards are achieved efficiently and safely. Methods analysis and related techniques are useful in office environments as well as in the factory. Methods techniques are used to analyze the following (Heizer et al., 2000, p. 394-396):
1. Movement of individuals or material. Analysis for this is performed using flow diagrams and process charts with varying amounts of detail.
2. Activity of human and machine and crew activity. Analysis for this is performed using activity charts (also known as man-machine charts and crew charts).
3. Body movement (primarily arms and hands). Analysis for this is performed using micro-motion charts.
Labor Standards and Work Measurements
Effective operations management requires meaningful standards that can help a firm todetermine the following (Heizer et al., 2000, p. 408-420)
1. Amount of labor contribution for any product (the labor cost).
2. Staffing needs (how many people it will take to meet required production).
3. Cost and time estimates prior to production (to assist in a variety of decisions, from cost estimates to make or buy decisions).
4. Crew size and work balance (who does what in a group activity or on anassembly line).
5. Expected production (so that both manager and worker know what constitutes afair day’s work).
6. Basis of wage-incentive plan (what provides a reasonable incentive).
7. Efficiency of employees and supervision (a standard is necessary against which to determine efficiency).
Properly set labor standards represent the amount of time that it should take an average employee to perform specific job activities under normal working conditions. The labor standards are set in by historical experience, time studies, predetermined time standards and work sampling.
Historical Experience
Labor standards can be estimated based on historical experience i.e. how many labor hours were used to do a similar task when it was done last time. Based upon this experience the new time will be fixed for any new operation or works. Historical standards have the advantage of being relatively easy and inexpensive to obtain. They are usually available from employee time cards or production records. However, they are not objective, and we do not know their accuracy, whether they represent a reasonable or poor work pace, and whether unusual occurrences are included. Because their variables are unknown, their use is not recommended. Instead, time studies, predetermined time standards and work sampling are preferred (Heizer et al., 2000, p.409).
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