Eight samples of effluent from a pulp mill were each divided into 10 batches. From each sample, 5 randomly selected batches were subjected to a treatment process intended to remove toxic substances. Five fish of the same species were placed in each batch, and the mean number surviving in the 5 treated and untreated portions of each effluent sample after 5 days were recorded and are given in Table 5.20. Test to see whether the treatment increased the mean number of surviving fish. 

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Table 5.20 Data for Exercise 14. MEAN NUMBER SURVIVING Sample No. Untreated Treated 1 2 3 4 5 55 1 1.8 1 3.6. 5 5 5 1.2 4.8 5 16 55 7 2.6 4.4. 8 12