Category Archives: Work Performance Measurement

Work Sampling

Work Sampling

Whereas work measurement is concerned with how long it takes to perform a specific task or activity, work sampling is primarily concerned with how workers spend their time among several tasks or activities. For example. we may want to know how much time workers spend on indirect activities such as material handling to determine whether or not more cost-efficient material handling equipment should be purchased. In a hotel reservation call center, we would want to know what percentage of time is actually spent on the telephone. Work sampling' provides us with a method for determining the time spent on these activities, and involves observing a portion or sample of the work activity. Then, based on the findings in this sample, some statements can be made about how the employee or employees spend their time. For example, if we were to observe a fire department rescue squad 100 random times.

For example, if we were to observe a fire department rescue squad 100 random times during the day and found that it was involved in a rescue mission for 30 of the 100 times (en route. on site, or returning from a call), we would estimate that the rescue squad spends approximately 30 percent of its time directly on rescue mission calls. (The time it takes to make an observation depends on what is being observed, Often only a glance is needed to determine the activity, and the majority of studies require only several seconds' observation.) Observing an activity even 100 times. however. may not provide the accuracy desired in tile estimate. To refine this estimate. three main issues must be decided (these points are discussed later in this section. along with an example):

I. What level of statistical confidence is desired in the results?

2. How many observations are necessary?

3. Precisely when should the ob reservations be made?

The number of observations required in a work sampling study can be fairly large, raisins from several hundred to several thousand. depending on the activity and the desired degree of accuracy. The formula for computing the required number of observations.



N = Number of observations to be made.

Z = Number of standard deviations associated with a given confidence level.

p = Estimated proportion of time that the activity being measured occurs.

E = Absolute error that is desired.


Thus, with the above work sampling study. we can state that we are 95 percent confident that the true percentage of time that the clerks are idle falls within 3 percent of the study results. However, we don't always have an initial estimate of the proportion of time spent on a ,given activity (in fact, that is often why we are doing the work sampling study in the first place!). In these situations, we use p = 0.5 which will give us a worst case scenario. (If p is equal tq anything other than 0.5 we have, in effect, overestimated the sample size.) As an illustration, suppose in the above example we don't have an initial estimate for the proportion of time the front desk clerks are idle. In this case we would use p = 0.5, and the calculation of the sample size would be as follows:


The specific steps involved in conducting a work sampling study are

1. Identify the specific activity or activities that are the main purpose for the study. For example. determine the percentage of time equipment is working, idle, or under repair.

2. If it is possible. estimate the proportion of time of the activity of interest to the total time (e.g that the equipment is working 80 percent of the time). These estimates can be made from the analyst's knowledge. past data, reliable guesses from others, or a pilot Work sampling study. If no estimate can be made, assume. as stated above. that
the proportion is 0.50.

3. State the desired accuracy in the study results. 4. Determine the specific times when each observation is to be made.

4. Determine the specific times when each observation is to be made. If you are using an estimated time. recompute the required sample size at two or three intervals during the study period by using the data collected thus far. Adjust
the number of observations if appropriate.

The number of observations to be taken in a work sampling study is usually divided equally over the study period. Thus, if 500 observations are to be made over a la-day period. the observations are usually scheduled at 500/10, or 50 per day. Each day's observations are then assigned a specific time by using a random number generator. The need to divide the observations equally over the data collection period is even more important in service operations, where workers can be extremely busy during certain periods and less busy at other times.


There has been a long-standing argument that a large percentage of nurses'· time in a - hospital is spent on non-nursing activities. This, the argument goes, creates an apparent shortage of well-trained nursing personnel, a significant was of talent, a corresponding loss of efficiency, and increased hospital costs because nurses wages are the highest single cost in the operation of hospital. Further, pressure is growing for hospitals and hospital administrators to contain costs. With that in mind, let us use work sampling to test the hypothesis that a large portion of nurses' time is spent on non-nursing duties. Assume at the outset that we have made a list of all the activities that are part of Assume at the outset that we have made a list of all the activities that are part of nursing and will make our observations in only two categories: nursing and non-nursing activities. (An expanded study could list all nursing activities to determine the portion of time spent in each.) Therefore, when we observe nurses during the study and find them performing one of the duties on the nursing list, we simply place a tally mark in the nursing column. If we observe a nurse doing anything besides nursing, we place a mark in the non-nursing column.


Solution We now can proceed to design the work sampling study. Assume that we (or the nursing supervisor) estimate that nurses spend 60 percent of their time on nursing activities. - Also assume that we would like to be 95 percent confident that the findings of our study are within the absolute error range of plus or minus 3 percent. In other words, if our study shows nurses spend 60 percent of their time on nursing duties, we are 95 percent confident that the true percentage lies between 57 and 63 percent. Using the above formula, we calculate that 1,025 observations are required for 60 percent activity time and 3 percent error. If our study is to take place over 10 days, we start with 103 observations per day.

To determine when each day's observations are to be made, we assign specific numbers to each minute and a random number table is used to set up a schedule. If the study extends over an eight-hour shift, we can assign numbers to correspond to each consecutive minute. The list in Exhibit S10.8 shows the assignment of numbers to corresponding
minutes. For simplicity, because each number corresponds to one minute, a three-number scheme is used, with the second and third numbers corresponding to the minute of the hour. A number of other schemes also would be appropriate. (If a number of studies are planned, a computer program may be used to .generate a randomized schedule for the observation times.)

If we refer to a random number table and list three-digit numbers, we can assign each number to a time. The random numbers shown in Exhibit S 10.9 demonstrate the procedure for seven observations. This procedure is followed to generate 103 observation times, and the times are rearranged chronologically for ease in planning. Rearranging the times determined in Exhibit SlO.9 gives the total observations per day shown in Exhibit SlO.lO (for our sample of seven).

To be perfectly random in this study. we also should "randomize" the nurse we observe each time (the use of various nurses mini miles the effect of bias). In this study. our first observation is made at 7: 13 A\f for Nurse X. We walk into the nurse's area and check either a nursing or a non-nursing activity, depending on what we observe. Each observation need be only long enough to determine the class of activity in most cases only a glance. At 8:04 AM we observe nurse Y. We continue in this way 1O the end of the day and the 103 observations. At the end of the second day (and 206 observations), we decide to check for the adequacy of our sample.


Let's say that we made 150 observations of nurses working and 64 of them not working, which gives 70.1 percent working. Again, using the formula given above, we calculate that the required number of observations is now 895. Inasmuch as we have already taken 206 observations, we only need to take another 689 over the next eight days or 86 per day. Thi recalculation of the sample size should be done several times during - the data collection period.

If at the end of the study we find that 66 percent of nurses' time is involved with what has been defined as nursing activities, there should be an analysis to identify the remaining 34 percent. Approximately 12 to 15 percent is justifiable for coffee breaks d personal needs. which leave 20 to 12 percent of the time that must be justified and compared to what the industry considered ideal levels of nursing activity. To identify the a-nursing activities. a more detailed breakdown could have been originally built into sampling plan. Otherwise. a follow-up study may be in order.

Elemental Standard Time Data

Elemental Standard Time Data

.Elemental standard-time data are obtained from previous time studies and codified in tables in a handbook or in a computer data bank. Such data are used to develop time standards for new jobs or to make time adjustments to reflect changes in existing jobs. They are more correctly viewed as normal-time data, because tabled values have been modified by an average performance rating, and allowances must be added to obtain a standard time. Calculating a time standard for a new job using elemental standard-time data tables entails the following steps:

1. Break down the new job into its basic elements.  Match these elements to the time for similar elements in the table.

2. Match these elements to the time for similar elements in the table.

3. Adjust element times for special characteristics of the new job. (In metal cutting, for example. this is often done by a formula that modifies the time required as a function of type of metal, size of the cutting tool, depth of the cut, and so forth.)

4. Add element times together and add delay and fatigue allowances as specified by company policy for the given class of work.

The obvious benefit of elemental standard data is cost savings in that it eliminates the need for a new time study every time there is a new job. This saves staff time and avoids disruption of the workforce. The main practical requirement of the approach is that the elemental data must be kept up to date and easily accessible

Time Study

Time Study

A time study is generally conducted with a stopwatch, either on the job site or by analyzing a videotape of the job. Procedural, the job or task to be studied is separated into measurable parts or elements, and each element is timed individually. After a number of repetitions. the collected times are averaged. (The standard deviation may be computed to give a measure of variance in the performance times.) The averaged times for each element are then added together, and the result is the observed performance time for the operator. However, to make this operator's time applicable to all workers. a measure of speed. which is expressed as a performance rating and which reflects how hard the observed operator is working. also must be included to "normalize" the job. The application of a rating factor provides what is called normal time.



Work measurement is particularly important when workers are paid by the amount of work they actually complete, which is also referred to as piece-rate. Using the above example, if the hourly rate for an employee is S 12.00 per hour. then that employee. if on piece-rate. would be paid as follows:

60/2.82 = 21.277 pieces per hour is the standard.

$12.00 per hour/21.277 per hour = $0.564/piece.

Thus. a person working at 100 percent of standard would earn $12.00 per hour; a person working at 110 percent of standard would earn $13.20 per hour major problem with the piece-rate system is that it tends to reward quantity at the ex- sense of quality. result, firms that have adopted the niece-rate system usually will only for good pans that are produced.

Before a time study is conducted. each task is broken down into elements or parts. Some general rules for the break  down of a task are I. Define each work element to be short in duration but sufficiently long enough so that each can be timed with a stopwatch .a.nd the time can be written down.

2. If the o operator works with equipment that runs separately-the operator performs a task and the equipment runs independently-separate the actions of the operator and that of the equipment into different elements.

3. Define any delays by the operator or equipment into separate elements. How many observations are enough? Time study is really a sampling process; that is, we take a relatively small number of observations as being representative of many subsequent cycles to be performed by the worker. A great deal of analysis and experience indicates that the number of observations is a function of cycle length and the number of I repetitions of the job over a one-year planning period.

Work Measurement

Work Measurement

The subject of work measurement for establishing time standards has been controversial line the days of Taylor. With the widespread adoption of Deming’s ideas, it has become the subject of renewed criticism. (Deming argued that work standards and quotas inhibit process improvement, focusing all of the worker efforts on speed rather than quality.)


Nevertheless. all organizations need some form of standard time estimates for planning and budgeting, and many companies use them with success in work design. as demonstrated in the UPS case at the .end of this chapter. It is therefore important to understand the basic industrial engineering methods used to set standards:

1. Tim~ study (stopwatch and micromotion analysis).

2. Elemental standard time data.

3. Predetermined motion-time data.

4. Work sampling.

Each method has its advantages over the others and has particular areas of application Exhibit S 10.7 lists these methods and relates them to a general class of jobs.

Workers Interacting with Other Workers

Workers Interacting with Other Workers

Increasingly, work in both manufacturing and services is being performed by teams. The degree of interaction may be as simple as one operator hading a another, or a complex as a cardiovascular surgical team consisting or doctor. nurse. an anesthesiologist. the


operator of the artificial heart machine. an X-ray technician, standby blood donors. and the pathologist (anJ perhaps a minister to pray a little).

To facilitate analysis of team efforts. an activity or a gang process chart is used to plot the activities of each individual on a time scale similar to that of the worker-machine chart. A gang process chart is usually employed to trace the interaction of a number of workers with machines of a specified operating cycle, to find the best combination of workers and machines. An activity chart is less restrictive and may be used to follow the interact.ion of


any group of operators. with or without equipment being involved. Such charts are often used to study and define each operator in an ongoing repetitive process, and they are extremely valuable in developing a standardized procedure for a specific task. Exhibit S LO.6, for example, shows an activity chart for a hospital's emergency routine in performing a tracheotomy (an operation for opening a patient's throat surgically to allow him or her to breathe). where detailed activity analysis is of major importance because any unnecessary delay could be fatal.