Re: accurate vs. precise
Tom, As an old old timer I cannot resist the temptation, with tongue firmly in cheek, to take you up on the meaurement of the diameter of a rod. Whether it is accurate or precise, I do not know, but to me a rod is a lineal measurement, and does not have a diameter. 72 points = 1 inch 3 barleycorns (in length) = 1 inch 12 lines = 1 inch 12 inches = 1 foot 3 feet = 1 yard 6 feet = 1 fathom *** 5 1/2 yards = 1 rod, (pole or perch). *** 40 poles 1 furlong 8 furlongs = 1 mile 3 miles = 1 league. Square poles were used in my working lifetime for the measurement of small plots of land (typically allotment gardens), but I expect they are all in this new metric system. Very confusing. As no doubt you know :- 1 sq. pole = 30 1/4 sq. yds. 40 sq. poles = 1 rood 4 roods = 1 acre. etc., etc.. I have never come across lines and barleycorns, but I came across a reference to them some years ago which claimed that the last time lines had been used as a standard measurement was was in an Air Ministry specification for the size of the brass buttons on Royal Air Force uniforms, and that a barleycorn, a third of an inch, still ingers on - tho' not perhaps by name now - as the measure still used in boots and shoes. The difference between a size ten and a size ten and a half shoe, it was said, is NOT half the difference in size betweena a 10 and a 11, but is a size 10 plus a barleycorn - a third of an inch! Apologies for any momentary distraction from more essential items, such as sundials. Rod Eden - Original Message - From: Tom Semadeni [EMAIL PROTECTED] To: Phil Pappas [EMAIL PROTECTED] Cc: sundial@rrz.uni-koeln.de Sent: 29 April 1999 20:12 Subject: Re: accurate vs. precise Yo John! Old Timers?! Elderly?.or on old time? or been on the list for a while?or? My distant history tells me this: Precision usually means the number of significant figures used to report a measurement. Accuracy usually means how close your reported measurement is to the truth. Try this scenario: Let's say that the diameter of a rod is measured, using accurate laser interferometry, to be 0. 50148 cm. You measure the diameter with a micrometer using its vernier scale and report the diameter (A) as 0.5025 cm . You then use a coarse pair of calipers and report its diameter (B) to be 0.5 cm. Measurement A is MORE PRECISE because it implies a preciseness of measurement 1000 times, or 3 orders of magnitude, more precise than Measurement B. But measurement A implies that the true value of the diameter is between 0. 50245 and 0.50255 cm while measurement B implies that the true value of the diameter is between 0.45 cm and 0.55 cm. So measurement B is MORE ACCURATE since the true value lies within the implied limits of the B measurement and outside of the A measurement limits. (There is a systematic error in the A measurement of about 0.001 cm, probably due to a non-zeroed anvil measurement.) Dial accuracy is affected by the accuracy of placement and orientation of all of the components. Dial precision is affected by the clever use of geometry, scaling, and interpolation lines to aid the eye to read the edge or body of a shadow precisely. John Shepherd's beautiful dial at: http://www.uwrf.edu/sundial/welcome.html demonstrates a clever trick to increase the precision of an accurate dial. The trick is shown explicitly at: http://www.uwrf.edu/sundial/images.html where the good Professor shows three close-ups taken sequentially at 9:58, 10:00 and 10:02 am Central Daylight Time. The eye/mind is very good at extrapolating to a point, much better than finding and reporting the edge of a fuzzy shadow! The short and long of it, John, is that you shouldn't use accurate, precise and old timer without proper explanation! It seems that there was a discussion on this list a long time ago about tricks to help the eye get increased precision of dial time. There must be some rules of thumb about shadow distance, contrast, color etc. that would help us in our quest for precision (assuming that our dial is free of the normal errors!). I, for one, would appreciate some pointers (pun) on the topic of enhancing precision. Cheers, and congratulations at being at the proofreading stage of your Manual. Good stuff. t Phil Pappas wrote: Hello Old Timers: I've got another knit-picky question for you all to ponder. But you're a rather knit-picky group, so I don't think you'll mind. In proofreading the new fifth edition of my Sundial Owner's Manual, when discussing sundials, I think that I mistakenly used the words, precise and accurate, interchangeably, as if they meant the same thing. John Carmichael Tucson tel: 520-696-1709
RE: accurate vs. precise
Speaking of barleycorns reminds me that one can have a lot of fun with units. My favorite combination has components atmosphere = 101,325 newton/m^2 yard = 0.9144 m barn = 1 x 10^(-28) m^2 Combining these we get the barn yard atmosphere = 9.265158 x 10^(-24) joule a unit of energy. Just to relate this to our everyday experience, I would like to point out that the barn yard atmosphere is also a convenient unit of temperature, lying just between the Fahrenheit and Celsius degrees. I once heard that the mass of the electron in pounds is exactly 2.00 X 10^-30, but I don't know whose pound you need to use to get this. (When the Germans say pfund, they mean half a kilo.) --Art Carlson
Re: accurate vs. precise
On Fri, 30 Apr 1999, Jim_Cobb wrote: why don't you Anglophones try the metric system? - fernando Perhaps you should consider us bilingual in terms of units. Technically inclined (and many other) Anglophones use both English and SI units with comfort, though we prefer one set for some applications and the other for others. I wouldn't suggest that someone who is bilingual should avoid using one of his languages... (just to relate this to a previous thread :-) Jim Indeed! Why, just last night, I was working with furlongs and fifths of seconds... Dave
Re: accurate vs. precise
On Fri, 30 Apr 1999, Fernando Cabral wrote: Now, I hate when I see something like 2 yards, 2 feet, 5 inches and (the stroke of mercy) 1/8 -- It takes me several seconds to figure out how tall that person is! Or when I see specifications such as: Torque wheel bolts to 50 foot-pounds (6.9449 Newton-Metres), which comes back to the issue of precision! By the way: does stroke of mercy make sense in English? Yes, but we really never translate it - from the French! Coup de grace Dave
Re: accurate vs. precise
On Fri, 30 Apr 1999, Fernando Cabral wrote: By the way: does stroke of mercy make sense in English? Yes, but we really never translate it - from the French! Coup de grace Now I am sure it makes sense. But if I were to say coup de grace I would be accused of suffering of francophilia. :-) - fernando Aaac!! Never!!!
Re: accurate vs. precise
Dear Rod I am glad you've diverted from the accurate vs. precise issue. You know, to me this is so confusing I though I could help myself bringing the terms into Portuguese. You know what? Things only got worse. I was completely uncapable of presenting my own comments about the issue. Especially because in my speech I've never cared with the possible distinction between these two terms. But now that you referred to these Imperial measurement units I can confortably say: why don't you Anglophones try the metric system? It is so easy to handle that even some one that can not argue about the difference between accurate and precise can use it with accuracy and precision. :-) - fernando A.R. EDEN wrote: Tom, As an old old timer I cannot resist the temptation, with tongue firmly in cheek, to take you up on the meaurement of the diameter of a rod. Whether it is accurate or precise, I do not know, but to me a rod is a lineal measurement, and does not have a diameter. 72 points = 1 inch 3 barleycorns (in length) = 1 inch 12 lines = 1 inch 12 inches = 1 foot 3 feet = 1 yard 6 feet = 1 fathom *** 5 1/2 yards = 1 rod, (pole or perch). *** 40 poles 1 furlong 8 furlongs = 1 mile 3 miles = 1 league. Square poles were used in my working lifetime for the measurement of small plots of land (typically allotment gardens), but I expect they are all in this new metric system. Very confusing. As no doubt you know :- 1 sq. pole = 30 1/4 sq. yds. 40 sq. poles = 1 rood 4 roods = 1 acre. etc., etc.. I have never come across lines and barleycorns, but I came across a reference to them some years ago which claimed that the last time lines had been used as a standard measurement was was in an Air Ministry specification for the size of the brass buttons on Royal Air Force uniforms, and that a barleycorn, a third of an inch, still ingers on - tho' not perhaps by name now - as the measure still used in boots and shoes. The difference between a size ten and a size ten and a half shoe, it was said, is NOT half the difference in size betweena a 10 and a 11, but is a size 10 plus a barleycorn - a third of an inch! Apologies for any momentary distraction from more essential items, such as sundials. Rod Eden - Original Message - From: Tom Semadeni [EMAIL PROTECTED] To: Phil Pappas [EMAIL PROTECTED] Cc: sundial@rrz.uni-koeln.de Sent: 29 April 1999 20:12 Subject: Re: accurate vs. precise Yo John! Old Timers?! Elderly?.or on old time? or been on the list for a while?or? My distant history tells me this: Precision usually means the number of significant figures used to report a measurement. Accuracy usually means how close your reported measurement is to the truth. Try this scenario: Let's say that the diameter of a rod is measured, using accurate laser interferometry, to be 0. 50148 cm. You measure the diameter with a micrometer using its vernier scale and report the diameter (A) as 0.5025 cm . You then use a coarse pair of calipers and report its diameter (B) to be 0.5 cm. Measurement A is MORE PRECISE because it implies a preciseness of measurement 1000 times, or 3 orders of magnitude, more precise than Measurement B. But measurement A implies that the true value of the diameter is between 0. 50245 and 0.50255 cm while measurement B implies that the true value of the diameter is between 0.45 cm and 0.55 cm. So measurement B is MORE ACCURATE since the true value lies within the implied limits of the B measurement and outside of the A measurement limits. (There is a systematic error in the A measurement of about 0.001 cm, probably due to a non-zeroed anvil measurement.) Dial accuracy is affected by the accuracy of placement and orientation of all of the components. Dial precision is affected by the clever use of geometry, scaling, and interpolation lines to aid the eye to read the edge or body of a shadow precisely. John Shepherd's beautiful dial at: http://www.uwrf.edu/sundial/welcome.html demonstrates a clever trick to increase the precision of an accurate dial. The trick is shown explicitly at: http://www.uwrf.edu/sundial/images.html where the good Professor shows three close-ups taken sequentially at 9:58, 10:00 and 10:02 am Central Daylight Time. The eye/mind is very good at extrapolating to a point, much better than finding and reporting the edge of a fuzzy shadow! The short and long of it, John, is that you shouldn't use accurate, precise and old timer without proper explanation! It seems that there was a discussion on this list a long time ago about tricks to help the eye get increased precision of dial time.
Re: accurate vs. precise
Jim_Cobb wrote: why don't you Anglophones try the metric system? - fernando Perhaps you should consider us bilingual in terms of units. Technically inclined (and many other) Anglophones use both English and SI units with comfort, though we prefer one set for some applications and the other for others. I wouldn't suggest that someone who is bilingual should avoid using one of his languages... (just to relate this to a previous thread :-) I'll buy into that... Just to prevent you from calling me prejudiced. To be sure, I don't have anything against the English units. They just make my life harder. When I dive I have to use PSI (most diving equipment will only have that); when I inflate my tires, the manual never agrees with the air pump. If one is metric the other is English. Pilots insist in using feet to refer to altitude (although Brazilian pilots will always add a translation into meter)... Now, I hate when I see something like 2 yards, 2 feet, 5 inches and (the stroke of mercy) 1/8 -- It takes me several seconds to figure out how tall that person is! By the way: does stroke of mercy make sense in English? - fernando -- Fernando Cabral Padrao iX Sistemas Abertos mailto:[EMAIL PROTECTED] http://www.pix.com.br mailto:[EMAIL PROTECTED] Fone: +55 61 321-2433 Fax: +55 61 225-3082 15º 45' 04.9 S 47º 49' 58.6 W 19º 37' 57.0 S 45º 17' 13.6 W
Re: accurate vs. precise
Dave Bell wrote: On Fri, 30 Apr 1999, Fernando Cabral wrote: Now, I hate when I see something like 2 yards, 2 feet, 5 inches and (the stroke of mercy) 1/8 -- It takes me several seconds to figure out how tall that person is! Or when I see specifications such as: Torque wheel bolts to 50 foot-pounds (6.9449 Newton-Metres), which comes back to the issue of precision! By the way: does stroke of mercy make sense in English? Yes, but we really never translate it - from the French! Coup de grace Now I am sure it makes sense. But if I were to say coup de grace I would be accused of suffering of francophilia. :-) - fernando -- Fernando Cabral Padrao iX Sistemas Abertos mailto:[EMAIL PROTECTED] http://www.pix.com.br mailto:[EMAIL PROTECTED] Fone: +55 61 321-2433 Fax: +55 61 225-3082 15º 45' 04.9 S 47º 49' 58.6 W 19º 37' 57.0 S 45º 17' 13.6 W
Re: accurate vs. precise
why don't you Anglophones try the metric system? - fernando Perhaps you should consider us bilingual in terms of units. Technically inclined (and many other) Anglophones use both English and SI units with comfort, though we prefer one set for some applications and the other for others. I wouldn't suggest that someone who is bilingual should avoid using one of his languages... (just to relate this to a previous thread :-) Jim --- -- | Jim Cobb | 540 Arapeen Dr. #100 | [EMAIL PROTECTED] | | Parametric| Salt Lake City, UT | (801)-588-4632 | | Technology Corp. | 84108-1202 | Fax (801)-588-4650 | --- -- My personal mission statement is I live up to the wild and vague promises made by salesmen.
Re: accurate vs. precise
Fernando wrote: Now, I hate when I see something like 2 yards, 2 feet, 5 inches and (the stroke of mercy) 1/8 -- It takes me several seconds to figure out how tall that person is! Now that's a tall specimen (2.57 meters)! By the way: does stroke of mercy make sense in English? - fernando No, I'm not sure of your meaning for that phrase in the context (though I have some guesses). English (at least in the U.S., I'm not sure about other locales) does have the phrase stroke of luck, but that's the only 'stroke' idiom that comes to mind. It also occurs to me to mention that being an anglophone is not at issue here. In the U.K. most units are metric. I think other commonwealth countries make primary use of the metric system. It is we in the U.S. who hold onto this 'English' system, which is ironic when one considers that the early U.S. pioneered using the decimal system in its monetary system, which I believed inspired the French after their revolution to make other measurements decimally based. Jim --- -- | Jim Cobb | 540 Arapeen Dr. #100 | [EMAIL PROTECTED] | | Parametric| Salt Lake City, UT | (801)-588-4632 | | Technology Corp. | 84108-1202 | Fax (801)-588-4650 | --- -- Great works are performed not by strength, but perseverance. -- Samuel Johnson
Re: accurate vs. precise
Dave Bell [EMAIL PROTECTED] wrote (I believe tongue in cheek): Indeed! Why, just last night, I was working with furlongs and fifths of seconds... Dave It's interesting that you mention this. Over the last several weeks I have been reading Tolkien's Lord of the Rings to my children. It contains references to furlongs, fathoms, and leagues. My children are able to relate to these units because they are simple multiples of units familiar to them. This is similar to bilingualism inasmuch as it makes excellent (and not so old) literature accessible. As long as some want to dismiss the English system as having a silly basis, may I propose we consider the hour, minute, and second. These are ridiculous units for the same reasons. Originally the second was one-sixtieth of one-sixtieth of one-twelfth of one-half of a day. At present the official definition of the second is goodness-who-can-recall-how-many oscillations of the light emitted by a certain atomic electron transition (again, who can recall which?). Yet, how many SI units are derived from the second? How much pain would be involved in incorporating Swatch Time in a new international system of units and using that to replace SI? In fact, at the time the metric system was developed (in revolutionary France) a decimal time unit was also developed (this has been discussed in this forum before. One could regard Swatch Time as a marketing revival of this concept.) It didn't catch on, and France eventually went back to the second. But it would have been easier for us to use now than the second had it caught on. Then we can move on to consider degrees, arcminutes, and arcseconds. We could do geography in radians! After that, how about monetary units? Legal systems? Language? Cultural norms? Hair length? Etc. We keep using these units because they are familiar and not too inconvenient, and the transition would be inconvenient. In the U.S. we continue to use the 'English' system for similar reasons; we continue to use inches for much the same reason we don't have robust professional leagues for soccer (er, football)--one we like and the other we don't much care for. Jim --- -- | Jim Cobb | 540 Arapeen Dr. #100 | [EMAIL PROTECTED] | | Parametric| Salt Lake City, UT | (801)-588-4632 | | Technology Corp. | 84108-1202 | Fax (801)-588-4650 | --- -- To question a wise man is the beginning of wisdom. -- German proverb
Re: accurate vs. precise
I think Mike Shaw stated the matter for good using fromerly cited examples of darts and guns. Strictly speaking accuracy and precision are sort of independent. If your mean (average) shot is on the target then you are accurate even if you never hit the target precisely. Your shots may be off the target by a large distance. You are accurate so long as the mean shot is on the target. If the dispersion of your shots is small then you are precise. Of course your shots do not have to converge on the target. So it may happen that you are extremely precise but not accurate like if your darts all converged on lower right corner within a radius of an inch. Using darts (or shooting) you may have examples of any compbination of accuracy and precision. Shooting examples are best because they clearly show what closeness of mean to target and dispersion (measured by standard deviation) mean. But concepts of accuracy and precision apply everywhere in life. Not surprisingly the distinction between the two is very clear in technology, navigation and gunnery even if terms are used interchangeably. I would add another remark. Precision is costly and you know it very well even from sundialing experience. Accuracy is related to your attitude and skills. Precision depends on apparatus and resources you have at hand. So if you are not accurate in general, then the best precision probably will not help you because you focus off the target or on the wrong target. As a rule we should be always accurate and as precise as we can afford it. Slawek At 10:44 PM 4/29/99 +0100, you wrote: Fire 10 shots from a gun at a target. If the 10 holes in the target are tightly grouped, but near the outer, you are precise but not accurate If they are loosely grouped evenly around the bull, you are accurate, but not precise. If you can get them tightly grouped around the bull, you are precise and accurate In mathematical terms: Accuracy is the closeness of the mean to the target figure Precision is indicated by the standard deviation I think Off to the annual BSS conference tomorrow for an entertaining weekend. Hope to see some of you there. Mike Shaw [EMAIL PROTECTED] 53.37N 3.02W Slawek Grzechnik 32 57.4'N 117 08.8'W http://home.san.rr.com/slawek
