 |
|
|
| Mean, Median and Average
|
|
January 30, 2003
|
|
|
Our careful readers have no hesitation bringing suggestions and corrections to our attention, even on matters that have no practical effect on navigation. We appreciate the input, and urge all our readers to catch our errors and misstatements, no matter how large or small, and let us know about them.
|
|
|
|
Dear Bob,
It was nice to meet you at the boat show last week. We are reading our new Waggoner as if we know nothing, because we intend to get out of familiar waters this year. Some of the information is indeed new to us, and all of it is useful. "Kill the wake," is a good example. I'll try that when next we go up to the boat.
In your discussion of differences between U.S. & Canadian charts you write "Since the U.S. chart datum has half the lower waters above it and half below it...." I thought Mean Lower Low Water is derived by taking the lower low water reading over a certain number of days, adding the readings and dividing by the number of days. Your description sounds like a median rather than a mean. For the heck of it, I counted the number of minus tides at Seattle this year and was surprised at how many there will be, but it was still only about 160. Could you clarify this please?
Thanks,
Eric Weissman
|
|
|
Response
|
|
|
|
Hi, Eric,
Oops! I'm not sure you're exactly right but I realize I was wrong. I've known since my days at Catherine Blaine Jr. High School that Mean, Median and Average are similar in many ways, but not the same. I got them mixed up when I wrote the charts piece.
My thick American Heritage Dictionary defines MEAN as halfway between the two extremes. MEDIAN is the middle point in a distribution, with an equal number of entries on each side. AVERAGE, as I understand it, would be the total of all the entries, divided by the number of entries.
Looking further, my 2002 Bowditch (American Practical Navigator) defines Mean Lower Low Water as "the average height of the lower low waters of each tidal day," usually as measured over a 19-year period. This definition seems to say that Mean and Average are the same, which doesn't help.
I think I'll rewrite that section to delete the half-above and half-below part, and simply say that some low tides are lower than MLLW. That's close enough.
I think both of us will agree that we're picking at nits here, but I appreciate your bringing it to my attention. I want the Waggoner to be as accurate as possible. Where complete accuracy isn't possible, at least it shouldn't be wrong.
--Bob Hale
|
|
|
Back to Readers Write In |
|
|
|
