8 STATISTICAL METHODS. 



proved Arithometer ($300 to $400). The "Brunsviga" cal- 

 culating machine (Herrn Grimme, Natalis & Co., Brunswick, 

 Germany, Manufacturers; price $140 to & 75) is highly recom- 

 mended by Pearson. 



To draw logarithmic curves and for the mechanical solu- 

 tion of arithmetical problems the instrument of Brooks 

 (Science, xvn, 690, not yet marketed) should be found useful. 



Precautions in Arithmetical Work. Even the 

 most careful computers make mistakes in arithmetical work. 

 It is absolutely necessary to take such precautions that errors 

 may be detected. The best method is for statistical workers 

 to compute in pairs, but absolutely independently, comparing 

 results as the work progresses, so that time shall not be 

 wasted by elaborate work done with erroneous values. In 

 case of disagreement both workers should recompute, start- 

 ing from that point of the work where their results check. In 

 cases where it is not feasible for the work to be done by two 

 people, it should be calculated on distinct pages of the note- 

 book proceeding through several steps on the one page and 

 then independently through the same steps on another page; 

 checking the work as it progresses. It will be found useful 

 as the work progresses to make rough checks by comparing 

 the results with the original data to see that the results are 

 probable. 



Neatness in arrangement of work and in the making of 

 figures is essential. It is best to make all calculations in a 

 book with pages about 20 cm. by 30 cm., quadruple ruled, 

 with about three squares to the centimetre, so that each 

 figure may occupy a distinct square. I like to work with a 

 pencil, of 2H grade, so that slight errors may be erased and 

 rectified. In case of larger errors running through several 

 steps of the work, the erroneous calculations should not be 

 erased but cancelled. 



In using logarithms with the six-place table given in this 

 book, it is ordinarily necessary to write the entire mantissa 

 to six places, and to determine the number corresponding to 

 any logarithm to at least six places by use of the table of 

 proportional parts given at the bottom of the page. Upon 

 the completion of the calculation the number of decimal 

 places cO be recorded will depend upon the probable* error of 



