Archive for July, 2010
Reaching your boiling point
Our marketing director emailed me this motivational video called “212° the extra degree.” this motivational video called “212° the extra degree”. It says that at this temperature water boils providing the steam needed to accomplish things. The idea is that only one degree of heat makes all the difference.
I get it. However, being a chemical engineer with an interest in being accurate about physical processes, I had to be troublesome by pointing out that here in Twin Cities at over 800 above sea-level the pressure drops enough that on average the boiling point drops to 210.5 F. But setting this aside and focusing only on the 1 degree between water and steam, one must keep in mind the huge difference of simply heating up water versus making it change state, the is, the heat (or enthalpy in technical terms) of vaporization.
Thank goodness that our marketing director had become accustomed to working with a bunch of engineers, statisticians and programmers who, when one asks “Could I talk with your for a minute?”, immediately set the timer on their digital watches for precisely 60 seconds (the the nearest one-hundredth).
Coincidentally, while vacationing in Wisconsin’s Door County, I enjoyed a fine demonstration of how hard it can be to bring a quantity of water to a boil. It’s a tradition there to throw a bunch of fish in one kettle and vegetables in another and cook them up with a wood fire. However, as I learned and experienced from a somewhat dangerous vantage point, a pitcher of kerosene provides the final heat needed to accomplish the boil-over. My eyebrows needed a bit of burn-back, so that’s OK.
What value for p is right for testing t (or tasting tea)?
Posted by mark in Basic stats & math, history on July 25, 2010
Seeking sponsors for his educational website, statistician Keith Bower sent me a sample of his work – this 5 minute podcast on p-values. I enjoyed the story Keith tells of how Sir Ronald Fisher, who more-or-less invented design of experiments, settled on the p value of 5% as being a benchmark for statistical significance.
This sent me scurrying over to my office bookshelf for The Lady Tasting Tea – a delightful collection of stories* compiled by David Salsburg.** Page 100 of this book reports Fisher saying that below p of 0.01 one can declare an effect (that is – significance), above 0.2 not (that is – insignificant), and in-between it might be smart to do another experiment.
So it seems that Fisher did some flip-flopping on the issue of what value of p is needed to declare statistical significance.
PS. One thing that bothers me in any discussion of p-values is that it is mainly in the context of estimating the risk in a test of the null hypothesis and almost invariably overlooks the vital issue of power. For example, see this YouTube video on Understanding the p-value. It’s quite entertaining and helpful so far as it goes, but the decision to accept the null at p > 0.2 is based on a very small sample size. Perhaps the potential problem (underweight candy bars), which one could scope out by calculating the appropriate statistical interval (confidence, prediction or tolerance), merits further experimentation to increase the power. What do you think?
*In the title story, originally told by Sir Ronald Fisher, a Lady claims to have the ability to tell which went into her cup first—the tea or the milk. Fisher devised a test whereupon the Lady is presented eight cups in random order, four of which are made one way (tea first) and four the other (milk first). He calculates the odds of correct identification as 1 right way out of 70 possible selections, which falls below the standard 5% probability value generally accepted for statistical significance. Salsburg reveals on good authority (H. Fairfield Smith–a colleague of Fisher) that the Lady identified all eight cups correctly!
**Salsburg, who worked for some years as a statistician at a major pharmaceutical company offers this amusing anecdote from personal experience:
“When I first began to work in the drug industry…one…referred to…uncertainty [as] ‘error.’ One of the senior executives refused to send such a report to the U.S. Food and Drug Administration [FDA]. ‘How can we admit to having error in our data?’ he asked [and]…insisted I find some other way to describe it…I contacted H.F. Smith [who] suggested that I call the line ‘residual’…I mentioned this to other statisticians…and they began to use it…It seems that no one [in the FDA, at least]…will admit to having error.”
Ink made to last and fonts that minimize its consumption
Over the past few weeks, I’ve come across a number of interesting inkles about ink.
- A team of U.S.-British researchers announced earlier this month that they deciphered previously-illegible scrawling by African explorer David Livingstone, which he made 140 years ago under desperate circumstances using the juice of local berries. See the image enhancement in this article by New Scientist Tech. Given the depressing content of Livingstone’s laments, it may be just as well he used ephemeral ink.
- The Dead Sea Scrolls, now on exhibit at the Minnesota Science Museum (see this picture, for example), were written with extremely durable black ink (well over 2000 years old!) comprised of lamp black (soot), gum Arabic and flaxseed oil. According to this Numerica entry on the chemistry of ink a red version was made by substituting cinnabar (mercury sulfide ? – HgS). That must have been used by the editor overseeing publication of the Scrolls. ; )
- Printer.com suggests that we all save ink by favoring certain fonts over others. For example Century Gothic* uses 30 percent less ink than Arial. As a general rule the serif fonts do better than the sans serif ones. An article by Dinesh Ramde of Associated Press on 4/7/10 reported that a school of 6,500 students, such as the University of Wisconsin-Green Bay, can save up to $10,000 per year by switching to an ink-stingy font. To really make a statement about their support for Earth, UW-GB ought to go with the “holey” ecofont. However, rather than going to something so ugly, perhaps the best thing for all concerned about going green would be to be prohibited from printing anything and just hand-write what’s absolutely essential to put on paper (or papyrus).
