Title: Chapter 10: Time Studies
1Chapter 10 Time Studies
- IE 5511 Human Factors
- Prof Caroline Hayes
2Time study topics
- What are they?
- What can you accomplish with them?
- What methods and equipment do you need?
- What do data sheets (for recording times) look
like? - How many observations do you need?
- How do you calculate allowances and standard
times (ST)?
3Time Studies
- Time studies are
- Observations of work and the time it takes to
perform it. - Method of determining a fair days work.
- Frederick Taylor popularized times studies in the
late 1800s. Founder of the modern time study. - Work is divided into elements which are timed.
4Time Study Methods
- Time studies can be conducted with simply,
low-cost equipment - Stop watch (or other time recording devices time
study board, computer, etc.) - Video and/or audio tape,
- Time study forms, and other written notes,
- Time study often combined with motion study (e.g.
additionally looks at how motions are made) - Early studies analyzed physical work, but many of
the principles/methods apply equally well to
analysis of cognitive work (e.g. using verbal
protocol studies.)
5Functions of Time Studies
- Establish work standards e.g. recommended
times in which tasks should be completed by
qualified, trained operators, without excessive
fatigue, - Set expectations which are fair to both employee
and company. - Identify sources of error, difficulties,
sub-optimal aspects, - Improve existing processes, tools, or work
environments,
6Functions of Work Standards
- Establish reasonable productivity targets for
experienced workers, - Provide productivity goals for training purposes,
- Eliminate waste,
- Make processes more consistent,
- Reduce variability, improve quality.
7Establishing Work Standards
- Need to use work measurement procedures (e.g.
time studies) to set accurate work standards. - Data must be specific to a particular
- process,
- work environment,
- tool set and
- operator population
- Estimates that are not based on data may not be
sufficiently accurate for setting standards which
have a large impact on company and employees.
8Preparing for a Time Study
- The steps in the process studied must already be
standardized e.g. sequences have been
determined. - Operator must be fully qualified, trained, and
acquainted with standardized process being
studied. - Must inform supervisor, union steward, department
head. - Make sure all materials are available for the
process.
9Time Study Procedure
- Select operator(s)
- Break task down into elements (before you start
study) - Observe operators performing task record time
taken for each element, over several cycles. - Assign appropriate allowances (e.g. allow time
for necessary but non-productive activities, such
as rest, cleaning eye-glasses, etc. - Determine appropriate work standards.
10Selecting an Operator
- Get supervisor to help in identifying appropriate
operators, - Ideally, you want someone qualified, trained and
very familiar with process (may need to provide
training before study) if your goal is to set
standards. - Prefer an average or slightly above average
operator. - Sometimes you have no choice of operator only
one person is available who does the job.
11Divide Task into Elements
- Work Element a group of motions that is relevant
to the experimenters study objectives. - (For cognitive work, divide verbal protocol
into utterances roughly equivalent to a single
thought.) - Watch for several cycles (before study starts) to
identify useful work elements for the task. - Look for easily identifiable start and end
signals, often auditory or visual. Examples - The clink of a part being set on the fixture,
- Setting a cup on the counter in front of the
customer, - The moment when a customers hand touches the
credit card as the cashier hands it back.
12Divide task into elements (cont)
- This is not so easy to do!
- Preparatory observations Devote a half hour or
so to observation of the task start to identify
relevant operations, and practice recording them. - Data sheets Create a spread sheet or recording
scheme to help you record elements quickly and
easily. - Work element revisions new elements may keep
popping up over several days! You may also find
that two or more elements should really be
combined. Example for cashiers, cleaning and
organizing, chatting with co-workers are just
different ways of waiting for customers. - Level of abstraction. The size of the divisions
between elements depend on what you need to do in
the analysis.
13Record Significant Information
- Time Study Observation form provides space for
- Study date
- Observer Name
- Operator Name
- Department,
- Study Start Time
- Study End Time
- Also useful to record
- Machines
- Jigs, fixtures
- Working conditions
- Sketch of work area layout
14Positioning Observer
- Stand slightly behind operator, dont sit
easier to follow movements of operator or get our
of way). - Try not to distract or interfere with operator.
- Avoid distracting conversation that may upset
routines.
15Divide Task into Elements
- Smallest unit that can be accurately timed is
about 0.04 minutes (approx 2 to 3 sec). - Breakpoints use sound and sight both to identify
breakpoints between elements, (e.g. sound of a
part clinking in finished bin, sound of a latch
clicking shut, etc.)
16Coffee study Goals
- To understand how long each activity took,
- To identify what typical processes were,
- To streamline processes, where possible,
- To set work performance standards, and customer
expectations, - How long should customers expect to wait for a
cup of coffee? - How should performance of stores be assessed?
- What performance goals should trainees aim for?
17Identifying work elements
- It can take several hours or days of observation
to identify all work elements and to come up with
a consistent naming. - New elements may keep appearing, over time,
18Two methods for recording element times
- Snapback method after recording the end of an
operations, snapback or reset the stopwatch to
zero. - Advantages dont need to compute element
duration, - dont need to record delays or foreign
elements. - Disadvantages may loose some time during
snapback - Continuous method Start timer at zero at start
of all observations, let it run continuously.
Record elapsed time at element breakpoints. - Advantages all time is recorded, operators and
unions like that, makes method easy to sell, - Disadvantages may take more computational effort
19Data recording sheets
- You may need to devise data recording sheets that
fit the study goals, the task and the type of
data. - You may use the example data recording sheets in
the book, but they are not meant to fit all
situations, - Examples
- Recording a fixed sequence of operations.
- Recording a variable sequence of operations,
- Recording arrival and wait times in a line,
20Recording a fixed sequence of operations
Repeated cycles of the sequence
Foreign Elements
21Examples of Data Recording Sheetsfor recording
operations that happen in an unpredictable order
custom assembly of one-off products
22Examples of data recording sheets for sampling
length of time customers wait in a line
23Other types of data
24How many cycles should be observed?
- There are several ways of estimating the number
of cycles that should be observed in order to
obtain accurate standard - The statistical method.
- The General Electric (G.E.) method,
25The Statistical Method Estimate numbers of
observations required
- Goal to limit the error in the estimate for the
mean operation time (OT) to plus or minus a given
percentage, k. - Equation to estimate n, no of observations
needed - n t s
- k x
- Problem If you havent taken any observations
yet, how can you know x and s ? - You cant. Must estimate them first with a small
pilot study.
2
26The Statistical Method Estimate numbers of
observations required
- Goal to limit the error in the estimate for the
mean operation time (OT) to plus or minus a given
percentage, k. - Equation to estimate n, no of observations
needed - n t s
- k x
- Problem If you havent taken any observations
yet, how can you know x and s ? - You cant. Must estimate them first with a small
pilot study.
2
27The Statistical Method Estimate numbers of
observations required
- Goal to limit the error in the estimate for the
mean operation time (OT) to plus or minus a given
percentage, k. - Equation to estimate n, no of observations
needed - n t s
- k x
- Problem If you havent taken any observations
yet, how can you know x and s ? - You cant. Must estimate them first with a small
pilot study.
2
28The Statistical Method Estimate numbers of
observations required
- Procedure it takes two steps to calculate sample
size - Pilot study Take small set of observations or
use historical data to estimate the parameters - Mean OT xp (mean operation time observed in
the pilot study), use xp as an estimate of x for
the full scale study - Sample standard deviation, s.
- Full scale study. Use these parameters to
calculate sample size of a larger study.
29ExampleEstimation of number of Observations
- Pilot study you take n 25 readings for an
element. You get 25 readings, x1 through x25
0.28, 0.24, 0.33, 0.33, etc. - When you summarize your data, you find
- xp S xi /25 0.30, where xp is the average
time required - to perform the work element.
- s S (xi xp)2 (.28-.30)
(.24-.30) 2 0.09 - v n 1 v
25 1 - Use s 0.09 from the pilot study to estimate s
for the larger study. -
30ExampleEstimation of number of Observations
- Pilot study you take n 25 readings for an
element. You get 25 readings, x1 through x25
0.28, 0.24, 0.33, 0.33, etc. - When you summarize your data, you find
- xp S xi /25 0.30, where xp is the average
time required - to perform the work element.
- s S (xi xp)2 (.28-.30)
(.24-.30) 2 0.09 - v n 1 v
25 1 - Use s 0.09 from the pilot study to estimate s
for the larger study. -
31ExampleEstimation of number of Observations
- Pilot study you take n 25 readings for an
element. You get 25 readings, x1 through x25
0.28, 0.24, 0.33, 0.33, etc. - When you summarize your data, you find
- xp S xi /25 0.30, where xp is the average
time required -
- s S (xi xp)2 (.28-.30)
(.24-.30) 2 0.09 - v n 1 v
25 1 - Use s 0.09 from the pilot study to estimate s
for the larger study. -
32ExampleEstimation of number of Observations
- Pilot study you take n 25 readings for an
element. You get 25 readings, x1 through x25
0.28, 0.24, 0.33, 0.33, etc. - When you summarize your data, you find
- xp S xi /25 0.30, where xp is the average
time required -
- s S (xi xp)2 (.28-.30)
(.24-.30) 2 0.09 - v n 1 v
25 1 - Use s 0.09 and xp from the pilot study to
estimates to jump start the calculation for the
larger study. -
33Example (continued)Estimation of number of
Observations
- Full scale study how many observations of an
element do you need to take in a larger time
study, in order be 95 confident that your
measurement of x is within k 5 of the true
value? - k 5 (acceptable error)
- a 1 confidence level 1 - .95 .05
- From pilot study we estimated xp S xi 0.30,
s 0.09 - Now you need to look up t. You can look up t if
you know a and the degrees of freedom (d.o.f) - d.o.f. np - 1 25 1 24
- n t s 2.064 x 0.09
153.3 observations - k x 0.05 x 0.30
(round up to integer)
2
2
34Example (continued)Estimation of number of
Observations
- Full scale study how many observations of an
element do you need to take in a larger time
study, in order be 95 confident that your
measurement of x is within k 5 of the true
value? - k 5 (acceptable error)
- a 1 confidence level 1 - .95 .05
- From pilot study we estimated xp S xi 0.30,
s 0.09 - Now you need to look up t. You can look up t if
you know a and the degrees of freedom (d.o.f) - d.o.f. np - 1 25 1 24
- n t s 2.064 x 0.09
153.3 observations - k x 0.05 x 0.30
(round up to integer)
2
2
35Example (continued)Estimation of number of
Observations
- Full scale study how many observations of an
element do you need to take in a larger time
study, in order be 95 confident that your
measurement of x is within k 5 of the true
value? - k 5 (acceptable error)
- a 1 confidence level 1 - .95 .05
- From pilot study we estimated xp S xi 0.30,
s 0.09 - Now you need to look up t. You can look up t if
you know a and the degrees of freedom (d.o.f) - d.o.f. np - 1 25 1 24
- n t s 2.064 x 0.09
153.3 observations - k x 0.05 x 0.30
(round up to integer)
2
2
36The t-distribution (pg. 701)Look up t-value in
the table (or use the Excel function)
Alpha, a
Degrees of freedom, d.o.f.
37The t-distribution (pg. 701)
Alpha 0.05
Alpha, a
Degrees of freedom, d.o.f.
t 2.064
d.o.f 24
38Example (continued)Estimation of number of
Observations
- Full scale study how many observations of an
element do you need to take in a larger time
study, in order be 95 confident that your
measurement of x is within k 5 of the true
value? - k 5 (acceptable error)
- a 1 confidence level 1 - .95 .05
- From pilot study we estimated xp S xi 0.30,
s 0.09 - Now you need to look up t. You can look up t if
you know a and the degrees of freedom (d.o.f) - d.o.f. np - 1 25 1 24. From
table t 2.064 - n t s 2.064 x 0.09
153.3 observations - k x 0.05 x 0.30
(round up to integer)
2
2
39The General Electric (G.E.) Method Assumes more
error in smaller measurements not much
attention to typical variability in the operation
itself)
40Foreign Elements
- A foreign element is one that does not explicitly
belong in the sequence - Typically one subtracts them from observations
(when possible) to get a more true time. - Examples
- Worker has to adjust glasses,
- Must speak to supervisor,
- Rest break, lunch break,
- Equipment search must find new wrench.
41Foreign Elements
- Some foreign elements can be eliminated,
- But others cannot or should not be
- Foreign elements can an idea of how much extra
time (e.g. allowances) is reasonable to allow in
an operation.
42Allowances
- Allowances refers to extra time allowed, beyond
completion of the task itself - Some allowances are necessary for health and long
term efficiency (like rest breaks), - Others are pragmatically necessary, (like time
for picking up dropped tools or consulting with
supervisor)
43Computing Standard Times
- A standard time is a combination of
- The time it takes to complete a task
- Allowances.
- This approach recognizes that it is not possible
to work at top efficiency all day, all the time.
44Methods for computing standard times
- Method 1 Add in allowances compute required
rest. - ST NT NT x allowance
- NT (1 allowance)
- Method 2 Compute allowances as a of task time.
- ST NT / (1 allowance)
ST Standard Time the time in which you expect
workers to complete an operation. NT Normal
Time time required to complete an operation for
a given operator OT Mean Observed Time to
complete an operation (from time study).
For an experienced operator who works at a 100
rate (R), OT NT, and NT OT x R/100
where R the performance rating of the
operator.
45Example Method 1
- Suppose that your time study shows that it takes
3.5 minutes on average to complete a task. Rule
of thumb for manual tasks 15 allowances. - ST NT (NT allowance)
- 3.5 min (3.5 min .15)
- 3.5 min 0.525 min
- 4.03 minutes.
- Experienced operators will be expected to
complete the task in this time.
46But how can you estimate allowances?
- Observe foreign elements what percentage of
total time do they comprise? - Chapter 11 outlines many additional methods for
calculating allowances - For personal needs,
- For fatigue reduction
47Next, identify possible sources of fatigue
- Abnormal posture,
- Muscular force,
- Ventilation,
- Lighting,
- Visual strain
- Mental strain,
- Etc.
- (see check list, Table 11 2).
48Question
- Does it make sense to estimate
- Allowances
- Standard time
- Efficiency
- for a cashier who may spend much time waiting for
customers to arrive?
49How should Standard Times be used to Evaluate and
Motivate People?
- What happens when you set up a reward system?
- All jobs have same standard time, but some are
more difficult, - Busy-time often results in slower production
because you are exceeding capacity, - Do you always get the behavior you expect?
50Class Exercise
- Perform task
- Do a pilot study of 4 observations
- Calculate how many observations for a full scale
study, a 0.5, k .20 - Pretend that your 4 observations ARE your full
scale study. - Calculate ST using Method 1.
- Let allowances 15
51Time Sheet
Date
Study start time
- Operation Start time End time
Total time - Average