Chapter XVIII

ON THE NATURE AND USE OF AN AVERAGE, AND ON THE DIFFERENT KINDS OF AVERAGE.

§ 1. _Preliminary rude notion of an average,_
2. _More precise quantitative notion, yielding_
(1) _the Arithmetical Average,_
3. (2) _the Geometrical._
4. _In asymmetrical curves of error the arithmetic average must be
distinguished from,_
5. (3) _the Maximum Ordinate average,_
6. (4) _and the Median._
7. _Diagram in illustration._
8-10. _Average departure from the average, considered under the above
heads, and under that of_
11. (5) _The (average of) Mean Square of Error,_
12-14. _The objects of taking averages._
15. _Mr Galton's practical method of determining the average._
16, 17. _No distinction between the average and the mean._
18-20. _Distinction between what is necessary and what is experimental
here._
21, 22. _Theoretical defects in the determination of the 'errors'._
23. _Practical escape from these._

(_Note about the units in the exponential equation and integral._)