806 



STATISTICS. 



known, that both posture and time of day 

 have a remarkable influence on the number of 

 the pulse. Or, to take another case, we wish 

 to ascertain the influence of some employment 

 upon health (say that of the letter- press 

 printer) ; but we overlook the important fact, 

 that in every printing office, two or three very 

 distinct occupations are carried on, of which 

 the most important are those of the com- 

 positor and pressman. Not being fully aware 

 of this fact, and of the wide difference exist- 

 ing between the two employments, we pro- 

 ceed to extract from some mortuary register 

 the ages at death of printers as a class, calcu- 

 late the average age at death, and then proceed 

 to group the whole class of printers with that 

 large class of occupations carried on in-doors, 

 with little bodily exertion, to which the com- 

 positor alone properly belongs, but from 

 which the pressman is, by the nature of his 

 employment, excluded. In this case, we 

 should have been misled by the common 

 name borne by men following two really dis- 

 tinct occupations, and our facts would again 

 cease to be comparable facts. A third and apt 

 illustration is afforded by the Asiatic cholera. 

 We wish to compare two different remedies 

 or plans of treatment ; but we administer the 

 one remedy, or adopt the one plan, at the 

 onset of the epidemic, and the other during 

 its decline. Here, again, our facts are not 

 comparable facts ; for it is one of the well- 

 known characteristics of this disease, that it 

 is more severe on its first occurrence than 

 during the period of its decline. The same 

 sort of error would be committed, if one 

 remedy were administered in an early, and 

 the other in an advanced, stage of the attacks 

 themselves. The principle which these illus- 

 trations are intended to enforce, is the ne- 

 cessity of selecting, as the elements of the 

 -same average, facts strictly comparable, or, in 

 other words, brought about by the same com- 

 bination of causes. Over the intensity with 

 which each cause acts in individual instances, 

 the observer can exercise no control. His 

 province is to ascertain that the same com- 

 bination of causes is at work to bring about 

 each phenomenon or event. If from ignor- 

 ance or oversight he fails in this duty, he im- 

 pairs the value of his facts, and vitiates his 

 inferences in proportion to the number and 

 force of the conditions that he has overlooked 

 or omitted. 



In the observation and collection, therefore, 

 of the individual phenomena or events which 

 are to serve as materials for our average re- 

 sults, the first precaution to be observed i?, 

 that those phenomena or events should be 

 strictly comparable as regards the combination 

 of causes by which they arc brought about ; 

 or, as the French statists express it, we must 

 ensure " I'invariabilttc de V ensemble des causes 

 possibles" The frequent omission of this most 

 necessary precaution has given birth to the 

 dogma of Morgagni Nan numerandcc scdpcr- 

 pendendce sitnt. obscrvationcs and to the most 

 valid objections urged against the applica- 

 tion of the numerical method in medicine. 



For the collection, arrangement, and classifi- 

 cation of the facts which are to form the ma- 

 terials of our averages, no concise rules can 

 be laid down. The tabular forms must adapt 

 themselves to the exigencies of each individual 

 inquiry ; and must be more or less compli' 

 cated as the subjects of investigation con- 

 sist of few or many particulars. In reporting 

 cases, for instance, and in collecting and 

 analyzing those recorded by others, tabular 

 forms embracing many particulars are re- 

 quired ; and the preparation of such forms 

 demands unusual skill and care.* The same 

 remarks apply to the collection and classifi- 

 cation of recorded experiences and opinions 

 bearing on particular subjects of inquiry f ; a 

 numerical summary of authorities favourable 

 and adverse to particular doctrines, consti- 

 tuting what may be not inaptly termed the 

 statistics of opinion. 



2. Of the average and extreme results de- 

 duced from observation. The observer having 

 exercised all due care in the observation of 

 his facts, having grouped together only those 

 events which owned the same combination 

 of antecedents or causes ; and having further 

 correctly performed the work of enumeration, 

 has thus obtained certain average and extreme 

 results, which are to constitute standards of 

 comparison and data for reasoning ; the 

 question naturally arises are these average 

 and extreme results sound and trustworthy 

 standards and data, or not ; and what are the 

 circumstances which render them the one or 

 the other ? Common sense and experience 

 combine to give an authoritative answer to 

 this question. Our average and extreme re- 

 sults are more or less sound and trustworthy, 

 as the individual facts from which they have 

 been calculated are more or less numerous. 

 Where the facts upon which it is attempted 

 to found a general principle, or to establish a 

 standard of comparison, are very few, we are at 

 once conscious of their insufficiency ; and the 

 more readily when an attempt is made to apply 

 the principle or standard in question to some 

 important practical purpose. A better illus- 

 tration of the futility of such an attempt can 

 scarcely be found than the well-known test of 

 Ploucquet. That author proposed to deter- 

 mine whether or not a child was still-born 

 by referring every doubtful case to a standard 

 of comparison, founded upon three observ- 



* On this subject the late Dr. Todd, of Brighton, 

 has written a very able work, which may be safely 

 recommended to all who desire to enter upon such 

 complicated investigations. The title of this work 

 is : _ The Book of Analysis, or a New Method of 

 Experience, whereby the Induction of the Novum 

 Organon is made easy of Application to Medicine, 

 Physiology, Meteorology, and Natural History ; to 

 Statistics,' Political Economy, Metaphysics, and the 

 more complex Departments of Knowledge. By 

 Tweedy John Todd, M. D., of the Eoyal College of 

 Physicians of London, &c. &c. 1831. 



f Reference may here be made to a paper pub- 

 lished in the 3rd volume of the Journal of the 

 Statistical Society of London, " On the best Method 

 of Collecting and Arranging Facts, with a proposed 

 New Plan of Common- Place Book." By the Author 

 of this Essav. 



