Chapter 10: Time Studies
IE 5511 Human Factors
Prof: Caroline Hayes
Time 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)?
Time Studies
Time studies are:
– Observations of work and the time it takes to
perform it.
– Method of determining a “fair” day’s work.
Frederick Taylor popularized times studies
in the late 1800’s. Founder of the modern
time study.
Work is divided into “elements” which are
timed.
Time 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.)
Functions 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,
Functions 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.
Establishing 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.
Preparing 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.
Time 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.
Selecting 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.
Divide Task into Elements
Work Element: a group of motions that is relevant to the
experimenter’s 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.
Divide 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.
Record Significant Information
Time Study Observation Also useful to record:
form provides space for:
– Study date – Machines
– Observer Name – Jigs, fixtures
– Operator Name – Working conditions
– Department, – Sketch of work area
– Study Start Time layout
– Study End Time
Positioning Observer
Stand slightly behind operator, usually
easier than sitting – easier to follow
movements of operator or get out of way).
Try not to distract or interfere with operator.
Avoid distracting conversation that may
upset routines.
Divide 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.)
Example
Caribou coffee study: Corporate Goals
Stated goals: To streamline operations so that
employees will have more time to interact with the
customers.
Additional benefits: customers will be happier if
they do not have to wait as long.
Example
Caribou coffee study: Analysts’ 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 trainee’s aim for?
Identifying 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,
Two methods for recording
element times
Snapback method: after recording the end of an
operations, “snapback” or reset the stopwatch to
zero.
– Advantages: don’t need to compute element duration,
don’t 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
Data 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,
Recording a
fixed sequence
of operations
Repeated cycles of
the sequence
Foreign Elements
Examples of Data Recording Sheets:
for recording operations that happen in an unpredictable
order: custom assembly of one-off products
Time and Motion Study. Site 2: Regular Machine Page
Barista: employee observations Friday, mo/day/year
T 2.35 3.56 4.31 5:04 5:23 5:42
Op Wash Fill Steam Wait Pour C Check M
T 5:55 6:14 6:21 7:55
Op Pour M Finish Place Super.
T
Op
Examples of data recording sheets:
for sampling length of time customers wait in a line
Time and Motion Study. Site 3: Drive Thru page ________
Coffee Order Line: customer observations Date ____, 20__
Entry clock Short description "X" if car No# already
time (in min (color, type: turns in first Order Clock
and sedan, station away part of Time (in min
second) wagon, SUV, from line line. and
when car pick-up, etc.) or exits seconds)
enters line line when car
or turns prematur stops at
away ely order kiosk.
Other types of data
Chanhassen: Customer Arrival Rates
Ave. for 1/30/06 and 1/31/06, 6:30 - 10:30 AM
Number# Entering Cashier Line
50
38 39 39
40
32
27 28
30
23 20
20
10
0
9
9
9
9
9
9
9
30
:5
:2
:2
:5
:2
:5
:5
0:
-6
-7
-8
-8
-9
-9
-7
-1
30
00
00
30
00
30
30
10
6:
7:
8:
8:
9:
9:
7:
Tim e
How 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,
The 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:
2
n= t s
kx
Problem: If you haven’t taken any observations yet, how can
you know x and s ?
You can’t. Must estimate them first with a small pilot study.
The 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:
2
n= t s
kx
Problem: If you haven’t taken any observations yet, how can
you know x and s ?
You can’t. Must estimate them first with a small pilot study.
The 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:
2
n= t s
kx
Problem: If you haven’t taken any observations yet, how can
you know x and s ?
You can’t. Must estimate them first with a small pilot study.
The Statistical Method
Estimate numbers of observations required
Procedure: it takes two steps to calculate sample size:
1. 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.
2. Full scale study. Use these parameters to calculate sample
size of a larger study.
Example
Estimation of number of Observations
1. 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 = Σ xi /25 = 0.30, where xp is the average time required
to perform the work element.
s= Σ (xi – xp)2 = [(.28-.30) + (.24-.30) + …]2 = 0.09
√ n–1 √ 25 – 1
Use s = 0.09 from the pilot study to estimate s for the
larger study.
Example
Estimation of number of Observations
1. 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 = Σ xi /25 = 0.30, where xp is the average time required
to perform the work element.
s= Σ (xi – xp)2 = [(.28-.30) + (.24-.30) + …]2 = 0.09
√ n–1 √ 25 – 1
Use s = 0.09 from the pilot study to estimate s for the
larger study.
Example
Estimation of number of Observations
1. 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 = Σ xi /25 = 0.30, where xp is the average time required
s= Σ (xi – xp)2 = [(.28-.30) + (.24-.30) + …]2 = 0.09
√ n–1 √ 25 – 1
Use s = 0.09 from the pilot study to estimate s for the
larger study.
Example
Estimation of number of Observations
1. 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 = Σ xi /25 = 0.30, where xp is the average time required
s= Σ (xi – xp)2 = (.28-.30)2 + (.24-.30)2 + … = 0.09
√ n–1 √ 25 – 1
Use s = 0.09 and xp from the pilot study to estimates to
“jump start” the calculation for the larger study.
Example (continued)
Estimation of number of Observations
1. 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)
α = 1 – confidence level = 1 - .95 = .05
From pilot study we estimated: xp = Σ xi = 0.30, s =0.09
Now you need to look up t. You can look up t if you know α and the
degrees of freedom (d.o.f):
d.o.f. = np - 1 = 25 – 1 = 24
2 2
n= t s = 2.064 x 0.09 = 153.3 observations
kx 0.05 x 0.30 (round up to integer)
Example (continued)
Estimation of number of Observations
1. 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)
α = 1 – confidence level = 1 - .95 = .05
From pilot study we estimated: xp = Σ xi = 0.30, s =0.09
Now you need to look up t. You can look up t if you know α and the
degrees of freedom (d.o.f):
d.o.f. = np - 1 = 25 – 1 = 24
2 2
n= t s = 2.064 x 0.09 = 153.3 observations
kx 0.05 x 0.30 (round up to integer)
Example (continued)
Estimation of number of Observations
1. 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)
α = 1 – confidence level = 1 - .95 = .05
From pilot study we estimated: xp = Σ xi = 0.30, s =0.09
Now you need to look up t. You can look up t if you know α and the
degrees of freedom (d.o.f):
d.o.f. = np - 1 = 25 – 1 = 24
2 2
n= t s = 2.064 x 0.09 = 153.3 observations
kx 0.05 x 0.30 (round up to integer)
The t-distribution (pg. 701)
Look up t-value in the table (or use the Excel function)
Alpha, α
Degrees of
freedom,
d.o.f.
The t-distribution (pg. 701)
Alpha, α
Degrees of
freedom,
d.o.f.
t = 2.064
d.o.f = 24
Example (continued)
Estimation of number of Observations
1. 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)
α = 1 – confidence level = 1 - .95 = .05
From pilot study we estimated: xp = Σ xi = 0.30, s =0.09
Now you need to look up t. You can look up t if you know α and the
degrees of freedom (d.o.f):
d.o.f. = np - 1 = 25 – 1 = 24. From table: t = 2.064
2 2
n= t s = 2.064 x 0.09 = 153.3 observations
kx 0.05 x 0.30 (round up to integer)
The General Electric (G.E.) Method
Assumes more error in smaller measurements – not much
attention to typical variability in the operation itself)
Using the data
from our in-class pilot study
Task: collating & stapling 3 sheets of paper
Operations:
1. Assemble sheets 1, 2, 3
2. Hand-off/Align/Staple
Can you the data from our in-class pilot study to estimate no#
observations needed to insure that we are:
– 95% confident (α = 0.05) that our answer is within:
– 10% error (k=.10)
Time Study Data Sheet
Process: Collating and Stapling
Day: Wed. November 17, 2010
Start time: 11:14 AM
Location: Room 108, Mechanical Engineering Bldg., Minneapolis, MN
cycle Operation 1 Operation 2
1 5 2
2 5 4
3 6 5
4 5 5
5 4 4
Average 5.0 4.0
StDev 0.7 1.2
Calculate n, sample size needed
for operation 1
xp = 5.0
s = 0.71
k = 0.10 (10% error); Let alpha = 0.05
n = t s 2 = ? *0.71 2
k xp .10 * 5.0
What value should we use for t?
Calculate n, sample size needed
for operation 1
xp = 5.0
s = 0.71
k = 0.10 (10% error); Let alpha = 0.05
n = t s 2 = 2.776 * 0.71 2 = 15.4 obs.
k xp 0.10 * 5.0
What if we decrease k to 5% ?
Calculate n, sample size needed
for operation 1
xp = 5.0
s = 0.71
k = 0.05 (5% error)
n = t s 2 = 2.776 * 0.71 2 = 62.7 obs.
k xp 0.05 * 5.0
The no# of observations greatly increases!
“Foreign” 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.
Foreign 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.
Allowances
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)
Computing 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.
Methods 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.
Example: 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.
But 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
Next, identify possible sources
of fatigue
Abnormal posture,
Muscular force,
Ventilation,
Lighting,
Visual strain
Mental strain,
Etc.
(see check list, Table 11 – 2).
Question:
Does it make sense to estimate:
– Allowances
– Standard time
– Efficiency
for a cashier who may spend much time
waiting for customers to arrive?
How 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?
Time Sheet
Date: Study start time:
Operation Start time End time Total time
Average: