1004 



VETEEINAEY HYGIENE 



have been operating through a length of time, representing 

 such events as ages, deaths, castings, destructions, etc., we 

 can ascertain, by means of an average, the mean value of 

 these collected figures. The average will not necessarily 

 give the value of any one unit of the group, for it is almost 

 sure to be either above or below the actual value of a single 

 unit; but it will give us a general value of the whole 

 series. For example, a business firm has 365 horses of an 

 average age of 8 years. It by no means follows that all or 

 any of the horses are 8 years old. Many will be above or 

 below— in fact, the ages of the horses may vary from 5 

 years to 20 years old. The quantities either above or 

 below the average are termed the 'extreme values.' As 

 Dr. Parkes expressed it, extreme values are the two ends of 

 the scale of which the average is the middle. 



Averages must not be struck from a series of percentages, 

 by adding them up and dividing by the number of series. 

 For instance : 



250 admissions with 10 deaths gives a mortality of 4-0 per cent. 

 120 admissions with 9 deaths gives a mortahty of 7-50 per cent. 

 1050 admissions with 35 deaths gives a mortahty of 3-33 per. cent. 



If the percentage mortality be added together and divided 

 by three, the result is 4-94, which is not the true per- 

 centage. 



The correct method is that 1,420 cases furnished 54 

 deaths, and the real percentage is, therefore, 3'8, and not 

 4'94, as calculated above. 



One more example may be useful : 



Monthly Admissions and Mortality for Diseases of the Digestive 



System. 



Digitized by Microsoft® 



