Page 169 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

148 Maritime Archaeology: A Technical Handbook, Second Edition
Precision—In simple terms precision is the closeness of duplicate mea- surements to each other. The precision of measurements is affected by random errors (usually poor reading of tape). Other errors include gross errors caused by transcription errors and systematic errors that are caused by a stretched tape or an offset at the zero end of the tape. The random errors are usually considered to affect the precision where the gross and systematic errors affect the accuracy.
Repeatability—Repeatability and reproducibility are subdivisions of precision; the first relates to measurements made by same operator and same equipment, the second the same measurements by different operators. Generally they refer to how capable a system is of produc- ing the same result when the measurements are repeated.
Normal distribution—A normal or Gaussian distribution is a commonly employed theoretical frequency distribution (Figure 4.29). Two stan- dard measurements of a frequency distribution are the mean (x¯) and the variance (s2).
Mean—The average of the values.
Root-mean-square—This is the mean of the square root of squared
values or, in other words, it is the mean of the values ignoring the sign. Variance The variance (s2) is defined as:
Where x is value, ¯x¯ is the mean, and n is the number of measurements.
Standard deviation—The standard deviation s is the square root of the variance, sometimes called the root-mean-square (RMS) value and is given by:
Figure 4.29 A typical normal or Gaussian distribution.
s2 =1Â(x-x2) n