v: PREFACE. 
for reading the greater part of the manuscript, and the proofs, 
and for making many criticisms and suggestions which have 
been of the greatest service, but also for much friendly help and 
encouragement without which the preparation of the volume, 
often delayed and interrupted by the pressure of other work, 
might never have been completed: my debt to Mr Hooker is 
indeed greater than can well be expressed in a formal preface. 
My thanks are also due to Mr H. D. Vigor for some assistance 
in checking the arithmetic, and my acknowledgments to Professor 
Edgeworth for the example used in §5 of Chap. XVII. to illustrate 
the influence of the form of the frequency distribution on the 
probable error of the median. 
I can hardly hope that all errors in the text or in the mass 
of arithmetic involved in examples and exercises have been 
eliminated, and will feel indebted to any reader who directs 
my attention to any such mistakes, or to any omissions, am- 
biguities, or obscurities. 
3. U.'Y. 
December 1910, 
ily