Sec. 4.2 



THE NORMAL FREQUENCY DISTRIBUTION 



91 



normal distribution the useful range of the X's is about six times the 

 size of the standard deviation. The reader should recall that this 



Figure 4.23. The normal frequency distribution curve for a population with 



ix = 60 and a = 10. 



approximate relationship between the range and the standard devia- 

 tion was used in Chapter 2 when the normal frequency distribution 

 was mentioned first. 



TABLE 4.22 



Coordinates of Points Satisfying Formula 4.22 for a Normal 

 Frequency Distribution with ix = 60 and a = 10 



X 



2/i 



X 



Vi 



Most students have asked (or heard someone else ask) an in- 

 structor: "Do you grade on the curve?" The curve which the stu- 

 dent has in mind is the normal frequency distribution curve; but it 

 appears from the discussion above that there is a different normal 

 curve for each combination of fx and a. This is correct; but the stu- 

 dent who is asking such a question is chiefly interested in his per- 

 formance relative to the other persons who took the same examina- 

 tion. He hopes that a grade of 40 on one test is as good as a grade 

 of 70 on another test if its relative rank among all grades on that 



