-42- 



everyday use. He suggests approximating the true distribution with a Chi- 

 square distribution of equal variance. Cochran (1977 p. 96) uses this 

 approach and defines the degrees of freedom of the approximating Chi-square 

 as^ 



DF 



eff 



i=l ^ 



i=i ^i"^ 



where g . 



"i ^^i " ^i^ 



S. = sample variance 



A more appropriate form for our purposes is given by 

 L 



DF 



eff 



ifl '^°^i 



L VOL . 

 i=l ^i- ^ 



The equivalence of the two formulas is shown in Appendix B, page 54. 



Using this result the appropriate value t is derived from the Student's 

 t distribution and the confidence interval is given by: 



VOL ± fSE. 



VOL 



