Today I read a headline that said “75% Probability Apple Stays With AT&T”. That was based on: “"couldn’t find compelling evidence" that AT&T's contract with Apple ends this year. He gives it a 50% chance. Additionally, there's a 25% chance that AT&T would bid for  and win  another year of exclusivity. Add them up, you get 75%”.
Whenever somebody adds up probabilities, that sets off an alarm in my head. What if the analyst believed that there was a 60% probability that AT&T would get the new contract? Then the sum of the probabilities would be 110%. That’s a little more than absolute certainty, which never exists in the stock market.
The lesson here is to use a little, what I will call, “senseitivity analysis”. Plug some plausible numbers in to make sure that the answer still passes a common sense test. We know that the probability must be 100% or less. In this case, it will certainly be less than 100%. So the result of 110% tells us that there is a math problem here. Doing a little “senseitivity analysis” is a good idea in simulation models as well as the stock market.
For the record, the correct answer is 62.5%. I will let you work out the math.
Thursday, February 4, 2010
Subscribe to:
Posts (Atom)
Popular Posts

Since the late 1990s, ExtendSim has had an embedded database as part of its simulation tool. Now in its second generation, it is so incredi...

I recently had a discussion with a scheduler at a manufacturing plant. I was trying to explain to him the benefits of simulation, and I thou...

I worked on a simulation study last year with Dr. Holt (University of Washington) and Dr. Srinivasan (University of Tennessee). The results ...

Over the last 10 years , more than a thousand students have attended one or more of my simulation classes. I have seen successful and not...

In my last blog, I listed some of the new features we are incorporating into ExtendSim 10. In this blog, I would like ...

Animation is an important feature of a simulation model. And it’s a great way to introduce a model to someone who has never seen a simul...

Have you ever been in line behind someone who pays with pennies? It’s not good is it? There’s time for counting, higher chance of error...