36 TROPICAL MEDICINE AND HYGIENE 



indication of the present danger in a certain house than 

 of a high malarial index in a village or country. It also 

 applies only to the time at which the examination was 

 made, for if susceptible mosquitoes are present in abund- 

 ance the introduction of one or two persons in whose 

 blood gametocytes are present may lead to the infection 

 of a large number of anophelines and transform a place 

 from one with a low to one with a high endemic index. 



Much careful work is yet required to ensure uniform 

 results in carrying out prophylactic measures cheaply. 

 Enquiries must include (i) the determination of the 

 dangerous and harmless species of anophelines present ; 

 (2) their breeding-places ; (3) the endemic index of 

 malaria as determined by various methods, and with a 

 sufficient number of observations to diminish the mean 

 probability of statistical error. 



Ross uses a modification of Poisson's formula by 

 Professor Karl Pearson, and illustrates the use by deter- 

 mination of the percentage error in the application of 

 the spleen test in a village as follows : 



Let N be the total number of children, n the number 



examined, and x the number with enlarged spleens. -^ X 



100 will be the percentage proportion with enlarged 

 spleens of those examined the spleen rate. 



We cannot conclude that the same rate will hold for 

 the entire number N. Let e denote the probable per- 

 centage error : 



o/ _ 200 / 2 x (n-x) I _ n-i 

 ~ n V ' n V H-l 



In the instance Ross gives (" Report on the Preven- 

 tion of Malaria in Mauritius ") 200 out of 800 children 

 in a village were examined, and the probable error was 

 + 8-65, so that, as regards merely the statistical error, 

 the rate, if half the children examined were found to 

 have enlarged spleens, would be for the whole infantile 

 population between 58-65 and 41*35, i.e., 50 8-65. 



