Jesse Liberty - Silverlight Geek

Quick Bits

Quick Bits - Items too large for Twitter, too small or off topic for the blog.

Updated frequently
(Listed newest articles first)

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August 13, 2009

A buddy at Microsoft leaked the following feature for Expression Blend:

 August 6, 2009

A few questions have come up about how the Turing Project threading works (now simplified) and about where the name and logo come from. All is answered in the new Turing Project FAQ.

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July 31, 2009 - Bayesian Probability

Spent the early hours of this morning reading a great blog post by Jeff Atwood which makes reference to a Elizer S. Yudkowsky’s Intuitive Explanation of  Bayesian Probability. The timing was good as I had just finished the (highly recommended) book The Drunkard’s Walk which agreed with Jeff that most humans simply are not wired to deal with probability very well.

Yudkowsky poses the following canonical problem:

The frightening thing is that according to Atwood, only 15% of doctors get this right. And they’re off by a lot. That is, the average answer is in the range of 80% while the correct answer is 7.8%.  Apparently, there is something about the way we think about the problem that makes 7.8% hard to accept, and Yudkowsky does a great job of walking you through the logic in painfully small steps.

To me, however, there is a pretty straight-forward way to think about this (though it may only be intuitive since I’ve been through this a few times).

What Do We Know & What Does It Imply?

We have three pieces of information:

1% of sample are TRUE  (that is have cancer)

80% of sample who are TRUE will test TRUE

9.6% of sample who are FALSE will test TRUE.

On the face of it, we should guess that the percentage of women who test TRUE who actually are TRUE (test positive and actually have cancer)  is pretty small based on two facts provided: the actual percentage of women from the sample who are TRUE (regardless of testing) is only 1%, and the test has a false positive for 9.6% of those tested.

So, my reasoning to solve this is:

1. Assume we have a sample of 1000 women (I use 1000 to reduce the amount I have to talk about fractional people, but I don’t use 10,000 as I get lost in the zeros).

2. We know that the reality is that of the 1,000 women, 10 will have cancer (1%).

990 = no cancer
10 = cancer

3. Of the 10 who have cancer, 8 will test positive
8 out 1000 women tested will test True and are True

4. Of the 990 with no cancer 9.6% will also test positive = 990 * .096 = 95.04. 
95.04 women out of 1,000 will test True but are False.

5. The total number testing true is 8 + 95.04 = 103.04. 
Of these, 8 actually have Cancer.

6. So the value for tests positive (103.04) versus is positive (8) is 8:103.4 or 0.773  or 7.8% 
(8 of the 103.4 = 8/103.4)

Not Being Misled

The key to this and many problems like it is to realize that what you are trying to find is the relationship between those who Test positive vs. the reality of those who are positive, which is why you need all three numbers.

Probability is Weird and Cancer Is Scary

True story: I have a friend who is the head of breast cancer surgery at a very respected hospital. I was at a dinner party where he mentioned that a “very small percentage of those who show up with a positive test actually have cancer.”  This caused a lot of confusion, and that is because we (a) don’t deal with probability well and (b) don’t understand policy tradeoffs as a result.

From a policy point of view, it is too expensive (money, time, etc.) to test everyone with a biopsy, etc.  The preliminary screening is sufficient to move your knowledge from the general 1% probability to the more specific 7.8%, which is enough to pursue. The fact that 20% of women with cancer will get a false negative doesn’t mean that it is better to test everyone, and the fact that out of every 1,000 tested 95 will falsely test positive (and be subjected to needless worry) also does not mean that it isn’t worth using the screening test.

That said, it would be good if, before you took the test, you were told that a positive result means you have less than an 8% chance of actually being positive and that in any case, the test doesn’t change the reality of whether or not you are positive!

This kind of confusion leads to people not flying to countries where there has been a terrorist incident but happily driving across country.

It is estimated that after the 9/11 attack, more people lost their lives by choosing to drive when they otherwise would have flown than were killed in the actual attack.

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July 25

Kindle:  iPhone and PDFs

I sold my Kindle 2 (took less than 30 seconds) and I now do all my ebook reading on the Kindle application on my iPhone. I don't mind the smaller screen, I like the navigation and form factor, and most important, all my books are in my pocket wherever I am.

There are, however, three limitations imposed by Amazon on the iPhone application, but I've overcome them and am posting this to tell you how. The limitations are:

1. No reading aloud
2. No subscriptions
3. No pdf files or other files emailed into your device

 My solutions to #1 and 2 may not be satisfactory to everyone. I use audible books on tape for reading aloud, I use other applications for subscriptions (more on that at a later date).  The key solution found this week is the pdf & other file formats.

 Getting PDFs to your iPhone

 Part 1 of this solution is to obtain Stanza. This is a great eBook application that runs on the iPhone and on your desktop

Stanza is a serious compeitor to the Kindle with access to commercial books and to free books and a great desk top reader. It will also convert from (virtually) any format to (virtually) any other, thus from PDF -> Kindle.  The problem is that while it is great at most things, it is a bit weak in the PDF-> Kindle conversion.

Enter calibre, an open source e-book management system that does a magnificant job with conversion

Here is a chopped down excerpt from a pdf file,

Stanza brought in all the text, but not the formatting quite as well as I would like

By adding Calibre, I'm able to get nearly a perfect rendering,

Gotta love it.

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July 8

Is the Book Wrong or Am I Just Confused?

If You want to think about it first, stop reading here.

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I believe this is not what he meant to ask but given that it is what he did ask,

My answer is R and 9. Reasoning:

• Turn the R over, if it is not 4 you have a counter example and proven the rule false
• Turn 9 over, if it is an R you have a counter example and proven the rule false

Turning over the 4 will not give you a dispositive answer. Here's why. Either it is an R or it is not. Assuming that he means the rule to cover only these four cards then if the flip side of the 4 is an R the rule is supported but if it is not an R the rule is not disproved, and that is what is essential.

The rule says “If a card has an R on one side, then it has a 4 on the other.”  It does not say “Every card with an 4 has an R on the other side” or the equivalent,  “only cards with Rs may have 4s”  Thus, having a T on the card neither proves nor disproves the given rule.

There is no need to test Q as it can’t have an R as each card has a letter and a number in the premise

His answer was R and 4. I think he’s wrong.  Am I missing something?

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July 4, 2009

The posting So Much Is Happening The Silence is Deafening briefly discusses what i personally believe is the most exciting new project for me since I began work at Microsoft – the Silverlight Open Source Project.  Full information is available from a link on the front page of my blog or through AgOpenSource.com

June 5 -

Just added a new feature: an on-going diary of my experiments in creating better videos through more advanced editing. You can find it here.

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--- May 18 ---

New feature: We've formalized the Mini-tutorials and given them a nifty logo:

Within a week or two we'll have an index of all the mini-tutorials making them easy to find, and I'll be ramping up the number I create. Feedback has been that they are a good middle ground between regular blog entries and full fledged tutorials.  More new features on the way as we get ready for Silverlight 3 RTW this summer.

--- May 17 ---

Quick Bits are recognized by two characteristics:

1. They are short
2. They may be somewhat off topic

I don't want to put them into the main blog (some folks really find them distracting while others like them), so my solution is that I will have a Quick Bits page for each month, with the latest entries on top.  Furhter, there will be a Quck Bits link on the sidebar of the main page with the date of the last update. Finally each month's Quick Bits page will have a link to the previous month's (except this one as it is the first).

From time to time a Quick Bit will just be a link off to an article or a rant on a page of its own. But not too often.

Hope it proves valuable. If not... well that's what's great about the web; you can try something new and kill it quick if its a dud.

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