VALIDITY OF THE SURVEY METHOD OP RESEARCH. 9 



making questions submitted to farmers conform to the terms in 

 which the farmer's knowledge exists. When this is done a proper 

 study of data furnished by farmers may reveal numerous important 

 facts never suspected either by the farmer or his questioner. For 

 instance, if in the farm-management survey made some years ago 

 in Lenawee County, Mich., the farmer had been asked directly what 

 the manure of a horse or cow was worth to him, he probably would 

 not have hazarded a reply. If he had it would have been little more 

 than a guess, not an estimate. But when the question was broken 

 up into its elements and he was asked to state the acreage and yields of 

 his various crops, the prices at which his products were sold, the 

 number and kinds of animals kept on the place, he answered readily 

 enough. By taking these data from many farms and comparing 

 those having relatively little stock with those having many, the 

 actual increment in crop values due to the manure of a single animal 

 was easily calculated. 1 



LAW OF ERROR. 



The law of error, frequently called the law of averages, may be 

 stated in many different ways. Perhaps as comprehensive a state- 

 ment of it as any is this : " Errors of measurement or observation 

 tend, in the absence of ' bias,' to group themselves about the true 

 value of the quantity measured in such manner as to eliminate each 

 other in the final average." 



The manner in which such errors group themselves about the true 

 average will be discussed in some detail a little later. 



Absolute accuracy is not obtainable in any kind of measurements. 

 In any case it is merely a question of degree of accuracy. 



The accuracy of any average depends on three things. First, and 

 most important of all, is freedom from " bias " ; that is, entire absence 

 of any tendency to make each measurement too high or too low. In 

 general, we have found bias singularly absent in practically all our 

 field studies of farm practice. It is true that some farmers deliber- 

 ately overestimate, but fortunately there seem to be about as many 

 who deliberately underestimate. These over and under estimates tend 

 to cancel each other and thus to reduce their effect on the resulting 

 averages. 



Second in importance is the number of items on which an average 

 depends. The larger the number the more reliable the average. The 

 reason for this lies in the fact that when a number of items is aver- 

 aged the larger the number the better the chance that any error will 

 be canceled by a similar error in the opposite direction. 



Since no measurement of any kind is absolutely accurate, every 

 measurement represents an error of greater or less magnitude. 



1 See Dept. Agr. Bui. 341, Table LX, p. 98. 



