166 SAMPLING NORMAL POPULATIONS Ch. 6 



The latter two are used quite commonly for estimates with 9 degrees 

 of freedom; the first inequality is used here chiefly for convenience of 

 illustration. 



As an application of the above-described methods, suppose that you 

 wish to learn what the average life of a certain type of light bulb is. 

 Suppose that ten sample bulbs of this type are left burning until all 

 have burned out, and the time it took each to burn out is recorded. The 



TABLE 6.32 



Outline of Some Samples from a Near-Normal Population, with /j, = 60 

 (Samples were taken by statistics classes from the population of Table 6.21.) 



Sample Confidence Limits on 



Number x s t 80% 90% 95% 



578 60.2 7.94 0.08 



Summary of 578 Confidence Intervals 



Limits Did Limits Did Not 



Confidence Include /x Include /x 



Coefficient Number % Number % 



.80 460 79.6 118 20.4 



.90 517 89.4 61 10.6 



.95 559 96.7 19 3.3 



