What a bad luck

This year, my Informs presentation is arranged at Wednesday again. I was looking forward to presenting my transshipment stuff in front of more people than the last year.

Two years in a row to present at Wednesday. What a bad luck! Considering that it is in Seattle, a lot of people living in the east coast would need to take flight in the morning so that they can get home before midnight. So if they don't want to cancel or miss two classes, they won't be able to make any Wednesday session.

Informs need to consider to shift their session starting Sat. instead of Sun. so that there is more people can attend all the sessions. Or they need to think out a way to enlarge the number of parallel sessions. Sun. to Wed. is not a good idea.

Stuck at Chicago

First trip, I got my bagage at second day after I arrived Chicago and got fever shortly.
Second trip, I got stomach flu at last day at Chicago.
This time, I am stuck at Chicago for two days due to bad weather.
Maybe Chicago knows I hate it and want to keep me as long as possible. Anyway, Chicago is beautiful right now and it is so easy to find a good restaurant with reasonable price.

Back to China for three weeks

I haven't been back to China for two years. Now it is time. I will departure on June 7th and be back on June 28th.

During those three weeks, I am also going to attend MSOM conference at Tsinghua. It is said that there is more people showing up in each session compared with Informs annual meeting. Let's see whether I can absorb more stuff during the meeting.

Also it is a good chance to talk with my old friends there. When I am in USA, I have very little connection to the industry. It is a big disadvantage when doing applied model for real business environment. One of my little sisters already opened a chain store. I hope to see how it operates daily, where my knowledge can help her, and whethter I can come up some new models.

When I am back to USA, I need to work on my proposal. Ideally, I propose in the middle of August. Though time is scarce resource to me, I will enjoy a short break

Open source codes of each paper

Usually when you read a journal paper and happen to to like it. So you want to implement their idea. Now the problem occurs, how can I implement those ideas? I need to work out every detail to work it out. Also only when you work it out in detail, you can say you understand it since you make computer understand it.

Why do all papers have their code attached? I will do it for all my papers from now on.

• to make my code more organized
• to make my work better known by others if they can see how concrete results come out
• to improve the work if I can get the feedback from others.

Back to Blog again

The first rule of Newton principle is that when you stop, you will stop forever unless there is a change of outside force. It can be applied to write blog. Once you stop, you will stop forever. But finally some force disappear. The spring semester is over.

Now I am rusty as a researcher. Too many things distract me. But I will be ready soon.

Lehigh IE rank drops 2 in 2007

Based on the latest US News, the rank of Lehigh IE drops to 15 from 13 this year. USC and NCSU go above Lehigh this year.

We need more professors!!

What does research do on airline industry

2007 airline quality rating comes out today. There are two major factors, I guess research field care about. One is on time performance which is related to scheduling problem. The other is deny rate which is related to RM.
http://www.eyewitnessnewstv.com/Global/story.asp?S=6312644
On time performance becomes worse

On-time performance, the report said, worsened last year, with 75.5 percent of
flights arriving on time, compared with 77.3 percent in 2005. JetBlue Airways
took a hit in February, when passengers on 10 planes spent from five hours to 10
1/2 hours sitting on runways at John F. Kennedy International Airport in New
York because of icy weather and gate congestion. It took days for the airline to
recover from the February storm and resume normal operations. It led JetBlue to
establish a customer bill of rights promising vouchers to passengers who
experience delays.

Deny rate (maybe more and more RM is adopted)

The study found an increase in the number of passengers bumped or denied
boarding because of oversold flights -- 1.01 denied boardings per 10,000
passengers last year, compared with 0.89 per 10,000 in 2005. JetBlue had the
lowest rate of bumped passengers; Atlantic Southeast the highest.

What should be my investment strategy?

Recently, due to my wife work, I get a chance to learn those retirement plans. So how should I manage the investment? My fundamental idea is to combine investment and your future life plan. There might be two big decisions. One is when to buy a house and the other is when to go back to China. The two financial risks associated are housing price and exchange rate. I am not expert in either area. But we still can do something to decrease the risk without too much effort.Generally you want to buy a house at low price and come back to China when dollar is strong. But what if the opposite things happen at the time when your decision is made. So at the current time, I put a certain percentage of money into real estate and international stock market. If hourse price goes high and dollar become weaker when I purchase a house and go back to China, it might be offset by the increase in those two funds. Although I cannot perfect hedge that since I cannot put all the investment into those two sources ( currently since I expect subprime loan would drive the housing price down, I put much less money into real estate fund ), it is still worth to do.

An inconvenient truth

Tonight I watch 'An inconvenient truth' again. It presents such a shaking fact - global warming. I suggest that everybody should watch this movie.

I should remind to turn down light every minutes.
Buy hybrid car for next new car.
Let everybody know TSP problem.

But when people tell you that our earth is going to destroy. What would you do? Take action to save it by controlling your desire or Do what ever you want to do before it is too late?

Trailer
http://video.google.com/videoplay?docid=2078944470709189270
http://www.climatecrisis.net/trailer/

Official website
http://www.climatecrisis.net/

Download
http://www.climatecrisis.net/downloads/

10 things should do
http://www.climatecrisis.net/pdf/10things.pdf

Chat Center

There has been enomous study on the call center. MS has special issue on the call center back to 2004. Last week, I found there is potential another variation of call center. I call it chat center. There is a problem after I reinstall my computer. I find email response is too slow. Therefore I choose to chat to solve my problem. I find it is effective way to solve complex problem rather than email and phone. After my case got solved, I begin to ask that representives some questions and get some answer:

Question: How many customer do you serve every day. just curious
Answer: It depends, but as we try to sort and help out each of our
customers, it is normally 12 to 15 customers on an average.
Question: Is it possible that you can serve two or more people at the same
time
Answer: Yes, at times when there are chats waiting we have to attend two
customers at the same time.
Question: As I remember there is checkbox at the beginning to let me import
what kind of questions do I have. Does that mean you can solve all the cases?
Answer: We have all the tools that are needed to resolve all the hardware
issue, every customers issue is different and sometimes almost same.
Question: Just another question, how do you balance between two or more
people at the same time?
Answer: We attend at the maximum 2 customers and that too only if the
volume of chats is too high.
Question: What if both people doing something else so that no one respond
you in time?
Answer: This gives me more time to search and try to find out more
solutions.

Usually each server in the call center can only serve one people. But in the chat center, each server can serve more than one people. But now we cannot assume the service time is exponential. Otherwise your previous service time is useless. It is more like CPU case. But CPU job is deterministic. The order of serve in the CPU may not affect system performance that much. But human being is a totally different story. Which model is good, how to get system performance, how should representives balance service to more than 2 people?

Another application of chat center is MSN messenger. Some companies use msn to serve their customer. This might bring another whole bunch of interesting issues. How to balace of messenger service and current work? When to use messenger, when to use phone?....

We should always keep our eyes open to our daily life.

Where is Killer Application of OM

Yesterday, I kept talking with different people. One is a graduated phd student from our department. He was used to work on inventory management in one Fortune 500 company. But the salary cannot compete with phd student from other field such as EE. We both feel that OM such as inventory theory (I doesn't mean those technology such as decreasing leadtime, since it has nothing to do with model) doesn't bring much improvement for the companies. For example, in the EOQ model, the model is quite insensitive to order quantity. And a lot of computation studies exclude cost such as purchase cost in order to emphasize the benefit of the model. Not to mention the restriction of the model. That might be low recognition of our field. I bet the number of people knowing IE would be less than 1/n knowing EE, ME... Why? Because those fields bring the more value to the society. People's live get changed by those fields.

Just like the slow progress of 3G, the lack of killer application obstructs development of 3G technology. The lack of killer application of OM/OR prevents smart people entering into this area and limits the amount of funding. We need more applications using OR/OM techniques to create great value to companies or even bring new companies.

Hope one day papers in our field are not just to improve the existence practice but also bring new model which companies would look into. Like CPLEX would send people to academic conferences. If more and more companies send their employees to OM ones. Also we have more master degree program other than MBA to train manager to have more quantitive competence. Then I would think our field gets recognized.

My research life approachs complete one more step

From winter break, I am doing on something which I feel very good. And I spend a lot of time to do literature survey to make sure no one did similiar thing before. Then I began to do proof and write a journal-like notes with 20+ pages. But yesterday, I read a paper published in top journal which is a special case as my idea. Although I did something else and proof is different, I still feel very very frustrated. I received Larry's comfort email. After one night sleep, I feel a little bit. And I also chat with my college friend who is top researcher in my mind. It makes feel better also. Our chat history is attached in the comments. Chinese is mine and English is my friend.

The power of the blog

As blog gets more and more popular, it becomes a source for the companies to advacate their products and improve their service. I remember there is a market research company which analyzes the blog posts and sell their analysis reports to its clients. Not it seems the companies take more preactive role here.

As I post the comments about dell product days ago. I receive the following email.

I am a customer advocate here at Dell headquarters in Texas. I found
your blogspot post about the problem you had with the motherboard on your Dell
Inspiron 600m (http://bsaint.blogspot.com/2007/03/balance-of-product-quality-and-warranty.html).
I wanted to check with you to see whether the replacement fixed the
problem. If you had any questions for me I would be more than happy to
answer them.

Today, the new motherboard is installed. It seems the problem is solved.

Responsible Authorship and Peer Review

James R. Wilson provides nice introduction about authorship and peer review.
When you write a paper, we should remember

If you’re doing an experiment, you should report everything that youthink might
make it invalid—not only what you think is right aboutit: other causes that
could possibly explain your results; and thingsyou thought of that you’ve
eliminated by some other experiment, andhow they worked—to make sure the other
fellow can tell they havebeen eliminated.

He also gives several things to check as a reviewer. Although I never review any paper, it is good to keep those questions in your mind when you write a paper.

1. Are the problems discussed in the paper of substantial interest? Wouldsolutions
   of these problems materially advance knowledge of theory,methods, or
   applications?
2. Does the author either solve these problems or else make a contributiontoward a solution that improves substantially upon previous work?
3. Are the methods of solution new? Can the proposed solution methods beused to solve other problems of interest?
4. Does the exposition of the paper help to clarify our understanding of thisarea of research or application? Does the paper hold our interest andmake us want to give the paper the careful reading that we give toimportant papers in our area of specialization?
5. Are the topic and nature of this paper appropriate for this journal? Arethe abstract and introduction accessible to a general reader of thisjournal? Is the rest of the paper accessible to a readily identified group ofreaders of this journal?
6. Are the clarity and readability of the manuscript acceptable? Is thewriting grammatically correct?
7. Does the manuscript contain an adequate set of references? Is adequatecredit given to prior work in the field upon which the present paper isbuilt?
8. Is the material appropriately organized into an effective mix of text,figures and tables? Are data given in tables better presented in figures orin the text?
9. Is the work technically correct? Are the main conclusions justified by theexperimental data and by logically valid arguments? Are the theoremsstated and proved correctly given the assumptions? In practicalapplications of the theoretical results, do the authors check the validity ofthe underlying assumptions?
10. Are there gaps in the discussion of the experimental methods or results?If there are such gaps, can the closing of these gaps be considered (i)essential, (ii) desirable, or (iii) interesting? Are the experimental methodsdescribed in sufficient detail so that other investigators can reproduce theexperiments?
11. Have the authors explicitly addressed the limitations of their study—thatis, have they adhered to Feynman’s ideal of “utter honesty” and “leaningover backwards” in reporting their results?

One more step closer to finish all the courses

I just submit my mid term take home exam for IP class. Now there are 2 final, 2 projects remaining before I finish all my desired courses to take. There are still some courses attracts to me. But I will only audit them and focus on doing research and make some progress.

A useful integral identity

$(\int_{t}^{T}\sigma(t,u)du)^2-\int_{t}^{T}\sigma (t,u)\Sigma (t,u)du\equiv 0$ where $\Sigma (t,T)=\int_{t}^{T}\sigma(t,u)du$
This is the identity I found when I derive bond price under Q measure in the homework.

Lattice Search

Suppose f(x,y) is a convex function. Now we want to find optimal integer x* and y* to minimize f(x,y) over integer set.
In continuou case, if we know x*(y), then we can just minimize f(x*(y),y) by taking derivative.
In discrete case, if we know x*(y), then can we say that -f(x*(y),y) is unimodular?

The balance of product quality and warranty

Today Dell agreed to change the motherboard for my laptop after I complained that my laptop sometime automatically turn off when I use battary only. I also change the touch pad last year due its insensitivity.

Almost 2 years ago, I buy this 600m with half price. In order to do that, I need to find a configuration above 1500. Maybe unlike other people, I only choose to increase meory then I select 4 year at home warranty, since I cannot trust dell's quality 100%. And I don't like play game and don't do computation in my laptop. I think this laptop can still be used until my graduation if there is no big problem. Now this decision is rewarded.

But why dell cannot improve its production quality when he knows this product has 4 year warranty. I remember it cost about 250$ to increase from 1 year to 4 year warranty. But the change of motherboard is not that cheap. Consider anther 2 years to go, dell may lost money in my laptop heavily.

Companies like dell which has the information of customer order before its production, should be able to determine the suitable quality for its product based on warranty duration.

Inside the box

'In my career, I've found that "thinking outside the box" works better if I know what's "inside the box". In music (as in life), we need to understand our pertinent history, and moving on is so much easier once we know where we've been' ---Dave Grusin (Award winning composer and jazz musician)

This is what I got from Starbucks cup yesterday.

XP to inventory problem

Here XP is the product of microsoft. X can be substituted by I, L, NL, D, S....
Yesterday, I try to read the paper about multilocation and multiperiod model by Karmarkar (1981). Originally when I saw programming form. I think I can handle it without too much difficulty considering quite amount of time I devote to those XP classes. But it turns out that I am still lost finally. That's why everyone need to focus.

But I think using XP especially constructing dual problem will help us increasing the understanding of the inventory problem more. In his paper, he shows that base stock is optimal policy under certain condition for example transshipment. And it follows monoticity properities.

Although I think there is a potential mismatch of his SNLP form and the transshipment application, I really like his method. At least it can be alternative method to show that base stock is optimal under single location without using DP and those derivative stuff. I think that we need to see more alternative methods to prove the same important stuff. Wish one day I can see a paper with name 'simple/alternative proof of xxx in inventory theory'.

If someday, there are erratum for those major journals. Although there is notes but that is not enough. Even the smatest and most rigorous people will make errors all the time. Also it would be very very nice to someone to discuss with you about the same paper that interest both. The understanding would be deeper. You will truely understand something when you teach/discuss with somebody. Also the new idea is easier to come out.

Why lost sales is still unsolved problem

These days, I read some papers about transshipment. There are two papers mentions why lost sales is hard to handle in general. This recall Zipkin's presentation about greatest unsolved problem in inventory theory. The reason is quite simple. If c(x) is convex, c(a(y)) is still a convex function when a(y) is affine function. But c([a(y)]^+) is not convex function in general. But it is still unimodal. After a little bit more thinking, another reason the authors not mention is that the sum of two unimodal may not be unimodal. That means unimodal doesn't perserve under expectation. So base stock policy may not be optimal policy. Hope I can a paper about lost sales to confirm this is the major reason to impede the discover of optimal policy form for lost sales.

OM in Health care

As my wife works at consulting firm for a pharmaceutical companies, I begin to know a little bit about the health care system in US now. It is quite surprising to me when I heard that american people spend every 1 of 5$ of their income in health care from the radio, although I know this industry is huge. Then the next question is natural based on my background. What our OM/OR people can do for health care industry. Obviously we cannot develop the new medicine to cure the cancer. But there is some ways that we can improve the efficiency, which is all what we can do.

The first example comes into my mind is that an example about the quantity of cancer treatment in an LP book. The second example is how to minimize the total walking distance in the hosptial. But those are all about optimization model. We definitely can do more. The impact might be even more important than other industries such as airline which is consider the hot research area but suffer the hard time now.

Experience in Starbucks

I have been in Starbucks frequently especially this week as a spring break. My working efficiency improves a lot since you can do nothing without internet access except the papers, books and winedit. I am quite satisfied with the environment there. You can stay all day long even if you just refill water after buying 1.85$ green tee. And when you are thinking, you can walk around. The more you go to the shop, the more familiar face you will see. Although there is no conversation, you still feel warm.

But today, I really feel tired at 3pm after finishing IP homework. However, there is no place for me to have a nap. I spend almost 2 hours to read all articles in today's NY time. From business to sports, from politics to alternative energy. This might be the first time that I read the newspaper so intensive after I leave China. I still remember the time that I fall asleep with whole bunch of business newspapers and magzines. I hope one day my english reading speed can be half as my chinese reading speed. Then I can be informed again.

The problem of unit in the relationship between backorder and waiting time

The formula for the relationship between backorder and waiting time is
V[B]=lambda E[BW] + lambda^2 V[BW]

I think there is exact same formula in queueing theory about the number in the queue and waiting time. But the puzzle to me is the match of unit. Suppose the unit of B, lambda, BW are item, item/day, day. V[B]=item^2, lambda E[BW] = item and lambda^2 V[BW]=item^2.

If it was me to derive this formula, I would throw it into garbage after this check. But sometimes, we cannot trust common sense.

The difference and similarity between Poisson and BM

The more I learn about BM, the more I feel there are strong connection between Poisson and BM. Last week, I ask a question. If we discrete Poisson process to small time interval, it behaves exactly as random walk with probablity lamda*h up by one and 0 otherwise. And counterpart of BM in discrete world is also random work with probability 0.5 up by 1 and probability 0.5 down by 1. Then I ask a question, what if we change the measure just like that we can change the BM with drift to BM without drift. Answer is obvious no, there are several differences.

• discrete BM, up and down probability is fixed. the only scale factor is step size. But the scale factor of poisson process is probability up and step size is fixed
• when we change the measure, we require that two probability measuresare equivalent which means the null space is the same. The null space of discrete one step of randome walke is R/{-1,1}. But poisson is R/{0,1}.

But when I ask Professor what is discrete case of BM with drift. Is it with the symmetric step size but different probability or different step size but the same probability or both or does not matter. I have not get answer yet.

IP homework is way too hard

The second set of homework takes me two days with 16+ hours to finish and without leaving a logic hole.
The last two logic gaps I filled just now are
1. LP relaxation of TSP formulation with subtour elimination constrainted is strictly contained in TSP formulation with subtour elimination constrainted replaced by $u_i-u_j-nx_{ij} \leq n-1$. This invovle how to scale up the problem then scale down.
2. Find as many affine independent points as possible for $P={x \in R^+ | \sum^n_{i=1} x_{ij}=1 for i=1,...n and \sum^n_{j=1} x_{ij}=1 for j=1,...n}$. It would be easy to see the rules by beginning with n=3 and 4.

In homework 1 of IP class, the following question is challenging.
1. show that $max{x_1 − \sqrt(2) x_2 | 1 \leq x1 \leq \sqrt(2) x_2 and x_1, x_2 is integer}$ is feasible and bounded, but has no optimal solution. I use the contradiction to show that there is no optimal solution.

uniqueness of the fixed point

In section 2.4.3 (Univalent mapping arguement) from 'game theory in SC analysis' by Cachon and Netessine, they claim that if best response function is one-to-one, then there is at most one fixed point(uniqueness of equilibrium). But why? It is possible that we can have x and y where x not equal to y such that x=f(x) and y=f(y). For example, f(x_1,x_2)=(x_2,x_1) is univalent mapping. But when x_1 = x_2=a in R, f(a,a)=(a,a). We can construct a trival game, player x and y write down a number, if x=y, then they both get 10, if not then their payoff is 10-(x-y)^2. In this game, the best response is f(x_1,x_2)=(x_2,x_1) .

Pros and cons of staying in a small university

The most advantage is that the relationship between prof. and students are close. But sometimes you will be shocked. There is only one student registering real analysis class, so that the class is cancelled. It is hard to imagine that this could happen to the basic graduate math courses in those bigger universities.

Very limited courses are offered in Lehigh. After my three years in Lehigh, there is no open course about heuristics, the stochastic process class only using introductory level of book 'Introduction to Probability Models' by Ross. It takes me waiting for 2 years to have IP open again. And there are many other regrets about courses in Lehigh.

More guns can be added into my powder magazine

Today, my wife gets and accepts her first offer. Now I can continue to my hobby to buy books. The books I plan to buy recently will be
Stochastic Calculus and Financial Applications
Nonlinear Programming: Theory and Algorithms
Theory and Practice of Revenue Management
An Annotated Timeline of Operations Research: An Informal History

Another new book in two days

Today my wife's sister comes to my home with book 'Real and Complex Analysis' I ordered from dangdang.com. It only cost me 39RMB (5 dollar).

Why Toy's R doesn't have sample for people to try

I also drop at Toy's R today due 25% gift certificate again. I try to get some gifts for my relatives and friends children. But there is no sample for most items for people to try except PS2 and Xbox 360. This setting loses the advantage to have a real store rather than online store only. Finally I get two idog mini which I already know what it is. But if I can try some other stuffs, I may buy more stuff. It seems that they don't know how to model customer behaviors

A new book comes to my book shelf

Today, I use 25% gift certificate of Border to get 'Game theory' by Fudenberg and Tirole. It is must have book for people doing game theory in SCM.

Crazy Tuesday and Thursday Next Semester

Finally I get my schedule for next semester. All the courses and meeting are concentrated in Tuesday and Thursday.
10:45-12:00 Advanced Stochastic Process II
12:10-1:00 IP Seminar (Th)
1:10:2:25 Real Analysis II
2:35-3:50 Financial Calculus II
4:00-5:00 Weekly Meeting with Larry(T)
7:00-8:15 Integer Programming
After crazy day, I will play tennis on Tuesday and Ping Pong on Thursday.

Wiki for Econ. Job Market

Is anything similar in IE or B-school.

Assignment of interview time slot

One of my friends from Econ. will graduate this year. He gets about 30 interviews during a conference at Chicago. Each interview lasts about half an hour. It is interesting to see how interviewees and interviewers agree on the time slot especially when there is no centralized system.

To queue or not to queue

It is not about the book To queue or not to queue. It is about my experience today.

Today, I sit in the Wendy's from 10:30am to 2:50pm to read and think. When I realized that I soon need to have lunch, there is waiting line. Then it becomes an optimization problem. My hungry degree increases by time. I don't want to waiting in the line waste my time not doing meaningful things. The length of waiting line is changing. What should I do?!

It turns out that I was poor to make that decision. I waited until that I cannot stand more and joined a long line. What is the form of best strategy under this situation? What if I have perfect information or not? Is any similiar situation in the real business situation?

Martingale and NP Complete problem

Last semester, I have two courses related to martingale. But how can it be used in inventory related problems. So far my limited literature reading hasn't touched any paper using martingale properties. I guess the reason is that it is not easy to find a martingale in inventory problem.

Today when I read 'The logic of Logistics', I found an application to use martingale difference sequence to develop average case analysis for Bin pack problem. Although it is not close related to inventory problems I am interests. It is a good start.

The reference is
Martingale inequalities and NP-complete problems

Wrong impression: any NP complete problem has no finite worst case bound for any heuristic

In network and graph class, we learn that there is no finite worst case bound for TSP. But I thought there is no finite worst case bound for any NP complete problem. Because of this wrong understanding, I argued with a speaker during Informs about his speech about bound of one multi-echolen problem. Of course, he cannot agree on my wrong opinion.

I realize this understand is wrong until today when I happen to read the bounds on Bin-Packing problem.