>From my recollection of the principles of experimentation, accuracy has to do with the magnitude of the spread of a series of repeated measurements of the same quantity around the mean or average measured value, representing something like the reproducibility of the measurement. The greater the accuracy the narrower the spread.
Precision has to do with how far the mean measured value is from the "correct" value. Of course there is a logical difficulty here. If you knew the "correct" or "true" value, why make any measurements? In the context of sundials, however, where presumably one can calculate or otherwise determine the correct time with great accuracy and precision, a sundial's indication of time can have both a random spread of values around the mean of a series of measurements of the same thing, and a fixed, error of that mean from the independently know true value. This also brings up a paradox. How do you perform a number of repeated measurements of 1:23:00 PM? If you cannot do this (because time is always changing on you), then perhaps we could compare our measurements only with the moving correct time, independently determined. In this case I suppose we could speak of the precision of each measurement being its departure from the correct time. If we repeat this measurement often over a period of time and discover that the time differences do not add to zero, then the non-zero amount would be an indication of the lack of precision of the measuremenet. The standard deviation of the time differences could be an indication of the accuracy. Does this make sense? -- Ross McCluney, Ph.D. Principal Research Scientist Florida Solar Energy Center, 1679 Clearlake Rd., Cocoa, FL 32922-5703 Voice: 407-638-1414 Fax: 407-638-1439 e-mail: [EMAIL PROTECTED] Florida Solar Energy Center: http://www.fsec.ucf.edu Sundials: http://www.sunpath-designs.com Introduction to Radiometry and Photometry: http://www.artech-house.com --------------------------------------------------------------
