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Apparel manufacturing is not a rocket science is often quoted by many to emphasizethe fact that apparel industry neither requires scientists nor use many scientific theoriesand techniques. Operations Research (OR) and/or Quantitative Techniques (QT) areinterdisciplinary branch of mathematics which use methods like mathematical modelling,statistics, and algorithms to arrive at optimal or good decisions in complex problems whichare concerned with optimizing the maxima (profit, faster assembly line, greater yield, etc.) orminima (cost loss, lowering of risk, etc.) of some objective function. In the field of operationresearch, there are many interesting scientific theories which are also taught in severalprofessional programmes in apparel production technology which may bring substantialbenefit to users. In his new series of articles, Dr. Prabir Jana, Professor, National Institute of

Fashion Technology, New Delhi explains some operation research tools, and how andwhere these tools can be used effectively.

PERT/CPM

Operation Research Application in Apparel Industry

Some of the common problems faced in apparel manufacturing are: How to estimate the duration of anyactivity; How to estimate the order lead time; How to identify activities with zero tolerance that requiresspecial attention to avoid delay in final delivery; How to identify and rationalize leave sa nction ofemployees to ensure minimum impact on performance or even prioritizing purchase of items forbottleneck activities. Is there any scientific technique, or is gut feeling and past experience the onlysolution? The Program Evaluation and Review Technique (PERT) and Critical Path Method (CPM), aretools in operation research designed to provide scientific solution to above queries.

While PERT was able to incorporate uncertainty in estimating task duration by making it possible toschedule a project while not knowing precisely the details and durations of all the activities, in CPM asingle estimate for activity time was used as if they were known or certain. While PERT is intended forvery large-scale, one-time, complex, non-routine projects with probabilistic time estimates, CPM issuitable for optimizing project lead time for regular repetitive projects with deterministic (certain) timeestimates. However, over the time PERT and CPM have been merged into a single techniqueconventionally referred to as PERT/CPM.

How does PERT/CPM Work?PERT charts depict events, activities, duration and dependency information to complete a project. Eachchart starts from an initiation node (which is also called an event) and from which the first activity, oractivities, originates. If multiple activities begin at the same time, they all start from the same node or

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, .other identifier, its duration, the number of people assigned to it, and in some cases the initials of thepersonnel assigned.

The other end of the activity line is terminated by another node also cal led event which identifies the startof subsequent activity.

While events consume no time, and use no resources, an activity is the actual performance of a task andhence it consumes time, requires resources (such as labour, materials, space, machinery), andrepresents the time, effort and resources required to move from one event to another. For example fitsam ple making is an activity that ends with an event fit sample sent for approval.

The estimated time (TE) taken to move from one event to another is calculated as the weighted average ofoptimistic time (O) or minimum poss ible time required to accomplish an activity, pess imis tic time (P) ormaximum possible time required to accomplish the same activity and the most likely time (M) timerequired to accomplish the activity (TE = (O + 4M + P) 6). As against this the Lead time is the time bywhich a predecessor event must be completed in order to allow sufficient time for the activities that mustelapse before a specific PERT event is completed.

Each activity is connected to its s uccess or activities forming a network of nodes and connecting lines. Thechart is com plete when all final activities come together at the completion node. When s lack time (excesstime and resources available in achieving this event) exists between the end of one activity and the start ofanother, the usual method is to draw a broken or dotted line between the end of the first activity and thestart of the next dependent activity.

In the PERT chart, the Critical Path is the longes t poss ible continuous pathway taken from the initial eventto the terminal event. It determines the total calendar time required for the project; and, therefore, any timedelays along the critical path will delay the reaching of the terminal event by at least the same amount.

Potential Application Areas in Apparel M anufacturingIncidentally like any project management, apparel manufacturing is also a network of activities executedsequentially and/or parallely. PERT/CPM can be applied very effectively to any stage in apparelmanufacturing.

Right from order receipt to delivery of goods, whole merchandising and manufacturing activities can beoptimized through PERT/CPM; in this case the critical path will decide the order lead time. But, in commonuse it is found that only pre-production activities, (i.e. from order receipt to start of cutting) or productionactivities (i.e. from cutting to goods s hipped out) are scheduled us ing this technique. Any new project, e.g.developing new collection for a season, expansion of factory capacity, setting up new facility or even a

performance improvement project by a consultant can be mapped and followed up using PERT/CPM fortimely outcome. For projects and activities that are happening for the first time, activity estimation can bedone by asking experts the optimistic, pessimistic and most likely time and calculating the expected time(TE). However for applications like in a sewing line (working in progress ive bundle system) accurate SMV(standard m inute value) of activities can be us ed as activity time to develop PERT/CPM and identify criticalsewing/non-sewing operations for monitoring production, WIP and abs enteeism in those operations toexercise s marter control.


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Pert Network

Activity code Activity description Predecessor Optimistic(a)Most

likely(m)

Pessimistic(b)TE

(a+4m+b)/6

a

Measurement chart

making - 2 4 6 4

bSampling fabricsourcing

- 3 5 7 5

c Pattern making a 4 5 9 5.5

d Marker making a 4 6 8 6

e Sample making b, c 4 5 9 5.5

f Labour costing d 3 4 8 4.5

g Material costing e 3 5 7 5Work Breakdown & Calculation of TE

SMV Conventional Line with NVA Operations

PERT/CPM Application in Sewing LineAs an example, the garment breakdown of shirt is first documented in tabular form with preceding andsucceeding operation written against each operation. The preceding operation for beginning sewingoperations (like front pocket hemming, collar runstitch, back-yoke join, etc.) is cut parts supply andmarked as nil. Once the table is com plete, a PERT chart may be drawn which shows that mul tiple sewingoperations starts simultaneously, then all converge to assembly. In the given example front and assemblysection is critical path and thus s hown in red colour.

Critical path helps calculate the realistic time that a garment will take to be completed. The sewingoperations in critical path are very important and cannot afford any delay, as every single minute delay inany critical path operation will delay the garment output also by same time. Operations in critical path

should be loaded with cut pieces on priority basis during style changeover.Supply of accessories to operations in critical path should be supplied on priority basis to avoid anypossible delay (Refer in detail SW May 2003 issue).

Implementing PERT/CPM in pre -production activitiesApparel pre-production consis ts of many activities, however to s impli fy the explanation, only few activitiesof apparel manufacturing are being considered. Whether one uses a software application or analyses theplan manually, one has to do the following steps to implement PERT:

Divide the project into smaller activitiesSequence and identify dependencies between activities , i.e. where one activity cannot start until another

has finishedEstimate a times cale for each activity (TE)Draw a PERT chart and indicate critical pathSlack time calculation


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Divide the whole project into individual activityTaking sampling as the project for which PERT chart is being constructed, the whole project is dividedinto seven activities and codified for easy reference hereafter (a to g in the following table).

Sequence and Identify dependencies between activitiesAll activities are sequenced, like which activity will follow next, and so on. Then relation/dependenciesbetween them are established, i.e. where one activity cannot start until another has finished. This isgenerally done in a tabular form (shown below), where codes of predecessors are written against everyactivity. Activities like Measurement chart making does not show any predecessor in the column;that means this activity does not depend on com pletion of any other activity and can s tart at the beginning.

There may be more than one activity in predecessors column; for example sample makingactivity cant start unless sample fabric sourcing (b) and pattern making (c) is completed,that is why it has two predecessors.

Estimate a timescale for each activity (TE)In above process every activity should have a predecessor and successor task. Secondly, estimatingtimescale of each activity is very important and one has to calculate the expected time by using theformulae described above. For example, to find out the expected time for pattern making activity,one can either conduct a cycle study to find out how much time that particular activity takes or ask theexpected duration of the activity from the concerned person. Then identify all necessary allowancesapplicable and categorize the allowances (allowances are the margin of time allowed in an activity) basedon avoidable and non-avoidable allowances. The cycle time without adding any allowance can beconsidered as optimistic time (O), add non-avoidable allowances with cycle time to arrive at most likelytime (M), and add non-avoidable as well as avoidable allowances with cycle time to arrive at pessimistic

time (P). If the activity is such that cycle time study is not possible then one can ask three personsassociated with the job what they think is the optimistic, most likely and pessimistic time and thenaverage out to arrive at estimated time (TE).

In the following example of sample making project there are seven activities, labelled a through g.Measurement chart making and sampling fabric sourcing can be done concurrently (a & b) while otherscannot be done until their predecessor activity is complete (pattern making cannot begin untilmeasurement chart is complete). Additionally, each activity has three time estimates: the optimistic timeestimate (a), the most likely or normal time estimate (m), and the pessimistic time estimate (b). Theexpected time (TE) is computed using the formula (a + 4m + b)/6.

The project review, approvals, user reviews, and so forth all take time and should never beunderestimated when estimating activities. It is not unusual for a review to take 1 to 2 weeks. Obtainingmanagement and user approvals may take even longer. When drawing up the plan, it is important to

include activities for documentation writing, documentation editing, project report writing and editing, andreport reproduction.

Drawing PERT chart and indicate critical path

Once these steps are complete, one can draw a PERT chart. The critical path is the path that takes thelonges t to complete. To determine the path times , add the task durations for all available paths.

The duration of path adf is 14.5 work days, that of path aceg is 20.00 work days and same of path beg is15.5 work days. So the critical path is aceg and the critical time is 20.00 work days; which means ifeverything goes as per plan sample making will take minimum 20 days to complete. It is important tonote that there can be more than one critical path (in a project more com plex than this example) or that thecritical path can change.

Slack time calculationThe slack of an event is a measure of the excess time and resources available in achieving this event.Positive slack (+) would indicate ahead of schedule; negative slack would indicate behind schedule; andzero slack would indicate on schedule. Start and finish are milestones and by definition have no duration;therefore they can have no slack. The activities on the critical path by definition have a slack of zero; e.g.LFa - EFa = 4 - 4 = 0.

Activity b has an LF of 9.5 and an EF of 5, so the s lack is 4.5 work days. Similarly, slack for activity d is 5.5work days and for activity f is 5.5 work days. Therefore, activity b can be delayed by 4 work days withoutdelaying the project. Likewise, activity d or activity f can be delayed 5.5 work days without delaying theproject (alternatively, d and f can be delayed 2.75 work days each). This has a very important bearing ondecision by merchandiser whose work is how much importance. Persons doing activity b, d, or f canactually take leave equivalent to slack time without any adverse effect on total sampling time of 20 days.However if any person in activity a, c, e or g falls sick by even half-a-day, total sampling time will be


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ex en e y a -a- ay.

Software solutionsPERT charts are usually drawn on ruled paper with the horizontal axis indicating time period divisions indays, weeks, months, and so on. It should be appreciated that software applications help in otherwisecumbersome manual calculations. Even though PERT applications are in-built with many apparelindustry specific ERP or planning softwares, there are several standalone free to use desktopapplications and open-source web-based applications available specifically for PERT/CPM applications.While these can be used as it is and customized for more user-friendliness . Several such software areKPlato, Open Workbench, TaskJuggler, Rational Plan, Concept Draw, etc. for desktop applications anddotProject, Eventum, Open Project Database, Project.net, ProjectPier for Open-Source web-based

applications.

ConclusionCommonly, spreadsheet applications (MS-Excel or equivalent) or flowcharting software (Microsoft Visio)are used as base for PERT/CPM network. Even though the chart may look alike, unfortunately theimportant functionalities like dependency, lag time etc. are not possible in Excel and thus such computerapplications are mere documentation rather than decision support systems. In a situation where apparelindustry executives are already overworked and pressed for time arriving at a rational decisionconsidering all alternatives is very difficult, and often decisions are taken merely based on pastexperience and gut feeling. PERT enables one to take rational scientific decision. Imagine that num ber ofpre-production activities for average order ranges from 50 to 60. Typically a group of 5-6 people(merchandisers, sampling in-charge, etc.) handle minimum 15-20 such orders at a time. The resultantnumber of options is beyond manual capability and only intelligent software programs can handle.

HistoryPERT model was invented by Booz Allen Ham ilton, Inc. under contract to the United States Department ofDefenses US Navy Special Projects Office in 1958 as part of the Polaris mobile submarine-launchedballistic missile project to analyze and represent the tasks involved in completing a given project,especially the time needed to complete each task, and identifying the minimum time needed to completethe total project. PERT was developed primarily to simplify the planning and scheduling of large andcomplex projects. During same time a research team led by Morgan R Walker and James E Kelly at E.I.Du Pont Inc. developed CPM network to improve planning, schedul ing and reporting.

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