Single tailed significance tests

Had the calculated value for t been less than 1.80 then there would have been no significance in the results and no apparent bias in the laboratory procedure, as the tables would have indicated a probability of greater than 1 in 10 of obtaining that value. It should be pointed out that these values refer to what is known as a double-sided, or two-tailed, distribution because it concerns probabilities of values both less and greater than the mean. In some calculations an analyst may only be interested in one of these two cases, and under these conditions the -test becomes single-tailed so that the probability from the tables is halved. 

Table 2.1 summarizes the single factor ANOVA calculations. The test for the equality of means is a one-tailed variance ratio test, where the groups MS is placed in the numerator so as to inquire whether it is significantly larger than the error MS ... 

Note-. Brain monocellular suspensions were dipped on the slides (per sample per slide) and subjected to single cell gel electrophoresis (SCGE). (See Section The Test Method for DNA Damage for detailed procedure.) Compared with the controls, the tail length of brain cells exposed to high fluoride, low iodine, and the combination of high fluoride and low iodine, increased significantly. 

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