了解如何提升工作效率和提高质量标准，学会分析和改善服务业或制造业商务流程。主要概念包括流程分析、瓶颈、流程速率和库存量等。成功完成本课程后，您可以运用所学技能处理现实商务挑战，这也是沃顿商学院商务基础专项课程的组成部分。

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了解如何提升工作效率和提高质量标准，学会分析和改善服务业或制造业商务流程。主要概念包括流程分析、瓶颈、流程速率和库存量等。成功完成本课程后，您可以运用所学技能处理现实商务挑战，这也是沃顿商学院商务基础专项课程的组成部分。

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From the lesson

第 2 单元 - 流程分析

您将在本模块中学习如何找到过程分析的关键因素：流速（flow rate）和流程时间（flow time）；如何找到瓶颈；如何优化劳力和库存；如何处理有多个流动单元的复杂情况。本模块教学结束后，您将能将运营分解为了流程，然后改进流程使利润和效率最大化。

#### Christian Terwiesch

Andrew M. Heller Professor at the Wharton School, Senior Fellow Leonard Davis Institute for Health Economics Co-Director, Mack Institute of Innovation Management

Many interesting operations are somewhat more complicated than what we can analyse

presently with our toolbox. One reason for this complexity is

that many processes need to serve multiple types of flow units.

Think of an Emergency Department. For example, there are trauma cases,

there are severely sick patients, and then there are patients that are just mildly sick.

Also, think about our Subway restaurant example.

Some people might just want to have a cookie, others want a coffee and a sandwich,

and yet others want just want to order for the entire family.

The consequence of that is that the processing times that the customers require

when they go through the process, they might differ for each and every particular customer.

Secondly, even the path that these

customers take through the process flow diagram might differ by customer.

In this session, we will introduce a more general way of finding the bottleneck and

determining flow rate, assuming that there's going to be a mix of flow units

that's going to journey through the process.

Consider the situation of a tax accounting firm.

The firm gets three types of cases come to their office.

Easy cases, of which they are arriving roughly 4 cases per hour.

Regular cases at a rate arriving at 11 per hour, and cases with foreign accounts,

they're arriving at a rate of 3 cases per hours.

Now, you notice here in the process flow diagram that while we still have the same

symbols of boxes and triangles capturing activities and inventory, that these three

different types of accounts are taking different paths through the process flow

diagrams. This is illustrated to you by use of color

with the GREEN arrows, capturing the flow for the foreign accounts, which is

different from the RED flow and from the BLACK flow.

How do we find the bottleneck in a process like this?

This is arguably somewhat more complicated.

We no longer can use our definition that the step with the lowest capacity is the

bottleneck. So, the reason that the condition no

longer works is that one particular resource might have little capacity,

but the flow solution might be such that very few flow units actually require service at

the resource. Let me illustrate how you find the

bottleneck in a situation like this. You take a look at each of the resources

in the process. Each and every one of them is a candidate

for being the bottleneck, which could be in this case, the first at Filing, the

Foreign Department, Department 1, Department 2, or the Printing

Department. Now, each of these resources has a

capacity which we just compute as always, the number of resources divided by the

activity time. For the first step, that is one over

three. This is expressed in applications per

minute which we can transform to 20 units per hour.

In the same way, we find that the capacity of the Foreign Department is 6 units per

hour, 12 at Department one, 15 at Department two, and 30 for Printing.

This is expressed in applications per hour.

Next, we ask ourselves, what's the demand for service at each and every one of the

five resources? Well, there are 3 types of demand here.

We have the foreign accounts, there are the regular accounts, and then there are

the easy accounts. Each of these 3 flow units is

contributing to demand into various resources.

If you look at the Filing Department, we have 3 units of the foreign accounts,

11 units of the regular accounts, and 4 units of the easy accounts

contributing to demand. This is equal to a total demand of 3

plus 11 plus 4 equals 18 units per hour.

At the Foreign step, this situation is different, because only the foreign units,

3 units per hour are going to arrive at this department.

There's no demand from the regular units and the easy units.

In Department one, we have the 3 units arriving from the Foreign Department, the

foreign units. We have the 11 regular ones, but we

don't have any ones from the easy. In Department two, however, we have no

foreign, no regular, just easy ones at a rate of 4 units per hour.

Finally, everybody shows up at printing, and the total demand there is 3 plus

11 plus 4 equals to eighteen. So, you notice here that a nice process

for diagram ideally using different colors to illustrate the different flows is going

to be very helpful as you do these calculations.

Finally, we can compute the ratio between the demand and the capacity as a sense of

busy-ness. We will call this measure the implied

utilization. Notice that this measure is different from

our utilization, which we defined as a flow rate divided by capacity.

Flow rate, by capacity, by definition, has to be less than 100%, less or equal to

100%. In contrast, implied utilization can well

exceed 100 % if there's more demand for a service than we have capacity.

In this case, we notice that 18 divided by 20 is the implied

utilization for filing. 3 divided by 6, it's a Foreign

Department. 14 divided by 12, it's

Department one. 4 divided by 15 at Department two,

and 18 divided by 30 at Department three.

We see that there's highest implied utilization, 14 divided by 12,

which is roughly 116.6%. This highest implied utilization makes the

Department one the bottleneck. The first approach was based on simply

adding up the flows at the various resources, computing a total flow, and

using that as our demand rate. The second approach I want to illustrate

is slightly different. Think about work flowing through the

system. At each of the resources, we have a

certain amount of work that the various resources can provide.

For example, at the Filing Department we are able to provide 60 minutes per hour of

time. And so, Foreign Department, we have two

persons working there and together their able to provide 120 minutes of work.

At Department number one, we have 3 people, creating a total amount of time

available of a 180 minutes, a 120 minutes for Department two, and then, a total of

60 minutes at the Printing Department. Now, ask yourself, how much work time will

be required by each of the flow units? Similar to the previous calculations,

we'll look at the 3 flow unit types. So, foreign ones, the regular ones, and

the easy ones. Now, we know at the Filing Department, we

have 3 units per hour arriving. Each of them will take 3 minutes of

work. Regular units, we have 11 units

arriving, each of them requiring 3 minutes of work.

And easy ones, we have 4 units arriving every hour, requiring 3 minutes of

work each. This creates a total workload of 54

minutes. So, the units here are really minutes of

work per hour. In the same way, we can compute that in

the Foreign Department, we have 3 units arriving, just as before, 3 units

arriving. Each of them corresponding to 20

minutes of work. This creates is a total workload of 60

minutes. There's no workload created in the Foreign

Department by regular units and the easy units.

Department one, we have 3 times 15 minutes caused by the foreign

cases, 11 times 15 minutes caused by the regular cases, and no work caused

by the easy cases. In Department two, we have only the work

from the easy cases of which there are 4 units an hour times 8 minutes per

unit. Finally, everybody shows up at printing.

Creating a workload of 3 times 2 minutes for the foreign cases, 11

times 2 minutes for the regular cases, and 4 times 2 units for the easy

cases. So, if we total these various rows, we see

that the workload in the Department one is 210 minutes.

It is going to be 32 minutes in Department two, and 36 minutes in the Printing

Department. Now, how do we find the bottleneck?

We simply compare the time that is requested for work at each of the

resources relative to the time available. This creates score of 54 divided by 60, 60

divided by 120, 210 divided by 180, 32 divided by 120, and 36 divided by 60.

Note that, these are exactly the same numbers as we have computed in the first

approach. For this reason, we can still call this,

the implied utilization. So, the numbers here are exactly the same

as they are before as far as the implied utilization is concerned.

And, you also see that these numbers here, in each of these columns, and these

correspond to what we had in the first approach.

The benefit of the second approach is that you have more flexibility.

For example, it might be that the easy cases take 2 minutes here at printing,

while the foreign account cases might take 5 minutes per case.

In other words, you can make the processing time contingent on the flow

units which you can only analyze with the second approach.

The second approach, however, comes at the expense that it is conceptually little bit

more difficult because you have to think about the generic flow unit being

1 minute of work. The very useful application of our new way

of finding the bottleneck, these were processes that have an attrition loss.

Consider the following example of the television firm.

The firm is looking for new series that they can air on television.

For that, they consider 500 ideas every year.

These ideas are pitched, 70 of these 500 are moved towards a script development.

These script then are reviewed, and the best twenty scripts are then moved into

pilot production. Out of these 20 pilots, 6 are turned

into new shows, and then the very best 2 are coming out as new series.

The computational finding the bottleneck in this process is very similar to what we

have done. It hinges on the basic idea that not every

one of these 500 ideas will all the way make it over to the end of the process.

For this reason, it is misleading to look at the capacity at each of these 5

steps and pick the lowest one as the bottleneck.

Instead, we should do a similar calculation as we did before.

This starts by, first, calculation, calculating the flow of ideas to each of

the five units. Only two of these 500 will make it all the

way to the new series production. Second, we're going to look at the

capacity of each of those five resources. And then, third, we will look at the

implied utilization as a ratio between the flow and the available capacity.

This will determine the location of the bottleneck.

Other examples of processes with attrition loss include underwriting processes such

as insurance and mortgage applications, as well as assembly processes or other

production processes that have quality problems, and where you have to scrap a

chunk of the flow units. What do you do if you have a mix of

products of flow units going through the same process?

One complication that we have seen is a flow unit might take different amounts of

processing time than another flow unit at particular resources.

Flow units might also differ in how they will journey through the process flow

diagram, as we have seen in the example with the attrition loss.

In this session, you've learned two methods of how to find the bottleneck in

such a system. Both methods are typically are based on

the concept of an implied utilization, which looks at the ration between the

workload at the resource relative to its capacity.

Now, I have to acknowledge that an interesting extension, that we didn't

cover in this module, is the case when you can adjust the product mix.

You can adjust the products mix to optimize some measures, such as profits,

by changing the mix of the percentages of the flow units.

This is something that requires some tools that are a little bit beyond what we can

do in this first course.

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