Work Systems and the Methods, Measurement, and Management of Work by Mikell P. Groover, ISBN ©2007 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. Learning Curves Sections: 1.Learning Curve Theory 2.Why the Learning Curve Occurs 3.Determining the Learning Rate 4.Factors Affecting the Learning Curve 5.Learning Curve Applications 6.Time Standards Versus the Learning Curve Chapter 19

Work Systems and the Methods, Measurement, and Management of Work by Mikell P. Groover, ISBN ©2007 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. Learning Curve Phenomenon Reduction in cycle time that occurs in a repetitive work activity as the number of cycles increases  An important topic in time study  When a worker accomplishes a task over and over, the time required for each successive cycle decreases as he or she learns the task  At first the learning effect is rapid, and the cycle time decreases significantly  As more and more cycles are completed, the cycle time reduction becomes less and less

Work Systems and the Methods, Measurement, and Management of Work by Mikell P. Groover, ISBN ©2007 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. Learning Curve Theory  According to theory, there is a constant learning rate that applies to a given repetitive task  Learning rate = the proportion by which the dependent variable (e.g., task time) is multiplied every time the number of task cycles doubles  Example: If T 1 = 10 hr and LR = 80%, then T 2 = 0.80(10) = 8.0 hr, T 4 = 0.80(8.0) = 6.4 hr T 8 = 0.80(6.4) = 5.12 hr and so on

Work Systems and the Methods, Measurement, and Management of Work by Mikell P. Groover, ISBN ©2007 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. Learning Curve for LR = 80%

Work Systems and the Methods, Measurement, and Management of Work by Mikell P. Groover, ISBN ©2007 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. Log-Linear Model When learning curve time data are plotted on log-log coordinates, the plot yields a straight line with slope m y = kx m where y = dependent variable, k = constant representing the value of the dependent variable for the first work cycle, x = number of work units completed, and m = slope m = and LR = 2 m

Work Systems and the Methods, Measurement, and Management of Work by Mikell P. Groover, ISBN ©2007 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. Same Learning Curve in Log-Log Plot

Work Systems and the Methods, Measurement, and Management of Work by Mikell P. Groover, ISBN ©2007 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. Two Widely Used Log-Linear Models 1.Crawford Model  Dependent variable is unit time T N 2.Wright Model  Dependent variable is cumulative average time

Work Systems and the Methods, Measurement, and Management of Work by Mikell P. Groover, ISBN ©2007 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. Crawford Model Most widely used learning curve model in industry today Example: T 1 = 10 hr, LR = 80%, how long will it take to complete the 20 th work unit? m = ln(0.80)/ln 2 = T 20 = 10(20) = 3.81 hr

Work Systems and the Methods, Measurement, and Management of Work by Mikell P. Groover, ISBN ©2007 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. Crawford Model  Total cumulative time  Cumulative average time

Work Systems and the Methods, Measurement, and Management of Work by Mikell P. Groover, ISBN ©2007 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. Wright Model  First learning curve model to be published  Based on cumulative average time per unit = T 1 N m  Total cumulative time: TT N = N or TT N = T 1 N m+1  Unit time: T N = TT N - TT N-1

Work Systems and the Methods, Measurement, and Management of Work by Mikell P. Groover, ISBN ©2007 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. Advantage of Log-Linear Model Consists of only two parameters: 1.The time associated with the first unit 2.The slope m, from which can be derived the learning rate LR  Conversely, if LR is known, the slope m can be deduced

Work Systems and the Methods, Measurement, and Management of Work by Mikell P. Groover, ISBN ©2007 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. Comparison of Crawford and Wright

Work Systems and the Methods, Measurement, and Management of Work by Mikell P. Groover, ISBN ©2007 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. Why the Learning Curve Occurs Contributions of the worker  Worker becomes familiar with the task - the worker learns the task  Worker makes fewer mistakes as the task is repeated  Hand and body motions become more efficient, and there is a rhythm and pattern developed  Minor adjustments in workplace layout to reduce distances  Fewer delays that interrupt the operation

Work Systems and the Methods, Measurement, and Management of Work by Mikell P. Groover, ISBN ©2007 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. Why the Learning Curve Occurs Contributions of the larger organization  Methods improvements by the IE Department  Fine-tuning of machinery and tooling  Development of special tooling  Technological improvements  Product design improvements  Improved quality of starting materials  Better scheduling  Improved logistical support  Better motivation of workers

Work Systems and the Methods, Measurement, and Management of Work by Mikell P. Groover, ISBN ©2007 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. Contributions to the Learning Curve Contributions to learning by the worker and the organization

Work Systems and the Methods, Measurement, and Management of Work by Mikell P. Groover, ISBN ©2007 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. Typical Learning Rates (Crawford) Type of work LR, % Assembly Prototype assembly 65 Clerical ops Inspection 86 Machining 90 Welding Industry LR, % Aerospace 85 Complex machines Construction Electronics mfg Machine shop Shipbuilding 80-85

Work Systems and the Methods, Measurement, and Management of Work by Mikell P. Groover, ISBN ©2007 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. Worker-Machine Systems  Learning in a work-machine system will be a composite of the learning rates of the two components of the system LR w-m = p w LR w + p m LR m where p w = proportion of cycle time controlled by worker, and p m = proportion of cycle time controlled by machine

Work Systems and the Methods, Measurement, and Management of Work by Mikell P. Groover, ISBN ©2007 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. Interruptions in the Learning Curve  Learning curve effect tends to be disrupted by interruptions in production  Possible reasons for interruptions:  Batch production, in which there are extended periods between repeat orders  Labor strikes  Vacations  Raw material shortages

Work Systems and the Methods, Measurement, and Management of Work by Mikell P. Groover, ISBN ©2007 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. Learning Loss due to Interruption

Work Systems and the Methods, Measurement, and Management of Work by Mikell P. Groover, ISBN ©2007 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. Other Factors Affecting Learning  Product complexity  More opportunities for learning - higher T 1 and lower LR  Preproduction planning  Fewer opportunities for learning - lower T 1 and higher LR  Labor turnover - slower learning (higher LR)  Plateau model - learning curve consists of two phases: 1.Start-up - log-linear model applies 2.Steady state - no further improvement

Work Systems and the Methods, Measurement, and Management of Work by Mikell P. Groover, ISBN ©2007 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. Plateau Learning Curve Model

Work Systems and the Methods, Measurement, and Management of Work by Mikell P. Groover, ISBN ©2007 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. Why the Plateau Model Occurs  In a worker-machine system, worker is finally limited by the speed of the machine  No incentives for workers to make further improvements  Management is unwilling or unable to invest in newer production technologies that would sustain the learning curve  Management does not believe that learning can continue indefinitely, and this becomes a self-fulfilling prophecy

Work Systems and the Methods, Measurement, and Management of Work by Mikell P. Groover, ISBN ©2007 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. Learning Curve Applications Accident prevention Capacity planning Cost accounting Cost estimating Cost reduction Contract administration Failure analysis Labor requirements estimation Make-or-buy decisions Methods analysis New product introduction Pricing negotiations Pricing strategies Product design changes Productivity analysis Quality control Reliability analysis Safety Vendor selection Wage incentives Warranty maintenance Worker performance analysis

Work Systems and the Methods, Measurement, and Management of Work by Mikell P. Groover, ISBN ©2007 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. Time Standards vs. Learning Curve  Apparent contradiction:  A time standard is a constant value that is defined under the following conditions:  Standard method  Standard work units  Performed by a trained average worker  Task is performed at 100% performance  But the learning curve predicts that the actual time for the work cycle will decrease over time

Work Systems and the Methods, Measurement, and Management of Work by Mikell P. Groover, ISBN ©2007 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. How to Reconcile the Contradiction  Although the standard time is a constant, the actual cycle time for a manual task is a variable  The time varies from one cycle to the next  One of the reasons why the cycle time varies is because the worker learns the task  The cycle time is gradually reduced as the number of repetitions increases, as predicted by learning curve theory  As long as there are no changes in methods or work unit, it seems fair to give credit to the worker for reductions in cycle time due to improvements in skill, effort, pace, etc.

Work Systems and the Methods, Measurement, and Management of Work by Mikell P. Groover, ISBN ©2007 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. Standard Reference Quantity (SRQ)  In batch production, it can be argued that the quantity of the production run should influence the value of the time standard  Surely a worker must be given an opportunity to learn a task before he or she is measured against the time standard  SRQ = the discrete quantity of production (e.g., 10, 100, 1000 units) for which the standard time for the task is applicable  If the actual quantity of production is different from the SRQ, then an adjustment in the time standard is appropriate

Work Systems and the Methods, Measurement, and Management of Work by Mikell P. Groover, ISBN ©2007 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. SRQ Adjustment Ratio