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FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



accuracy of the slopes, values of t may be selected for P=0.05, and by substituting 

 these in the equation, 



st 



values of 6-/3 may be calculated which, when added to b, or subtracted from it, will 

 give the limits of a range of slope values. The chances will then be 19 out of 20 

 that the true slope lies within this range. 



From these calculated ranges (table 11), it is clear that there was so little random 

 variability of the points about the lines of best fit, that mortality values are accurate 

 to within one or 2 percent per day for all segments other than A to C. 



There still remains the question: which of these combinations of straight lines 

 gives the most probably true series of survival rates? This may be investigated by 

 the formula for the significance of the difference of two slopes, again going through 

 the t test, using the formula 



t = 



6i — 6 2 



where 



V^Lsfe-xo^sfe-^) 2 ] 



^_ S(y,-Y l y+S(y 2 -Y 2 y 

 S ~ n'-4 



From the results given in table 1 1 , where the subscripts of b represent the initial and 

 terminal points of the segments, it is apparent: (1) That &a-c differs from 6 4 _ 8 just 

 enough to indicate that the survival rate probably is significantly higher in the larval 

 stages than in the egg, and therefore the two lines A-C and 4-8 better describe this 

 segment than the one line A-9. However, the latter does not differ significantly 

 enough from each of the former to preclude the possibility that it fairly well repre- 

 sents the general course of survival from the early egg stage to the 9-mm. larva. 

 (2) That 6n_22 is certainly significantly different from 6a-9, though not from 6 4 _ 8 . 

 The interpretation of these findings will be discussed in the following section. 



Table 11. — Estimates of accuracy of slopes of lines in figure 17 



Table 12. — Significance of the differences of the slopes of the lines fitted to various segments of the survival 



curve 



