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<br />nx and ny = number of samples in each data set <br />t(nx + ny-2), I-P = Fisher "t" statistic having nx + ny-2 degrees of <br />freedom aT the IOOP percent level of significance. <br />The following is The calculation for dissolved iron using data from TRA (x) and TRD <br />(y): <br />x = 0.368 y = 0.202 <br />nx = 12 ny = 12 <br />ax = 0.537 ay = 0.171 <br />The pooled variance is: <br />awl = (nx-I )ax2 + (ny-I )ay2 = I I (0.537)2 + I I (0.171)2 = 0.159 <br />nx + ny-2 22 <br />aw = 0.399 <br />• <br />yielding <br />t = (0.368 - 0.202) = 1.020 <br />0.399 I + I 12 <br />12 12 <br /> <br />The Fisher "t" statistic having 22 degrees of freedom at the 5 percent level <br />of significance is 2.07. Because t = 1.02 is less than 2.07, iT is concluded that the <br />difference between the Two means is not statistically significant at the 5 percent <br />level. The same conclusion is reached for manganese u_sing stations TRA and TRD <br />after editing the outlier value for July at TRD. <br />The remaining trace elements do not show increases in the downstream <br />direction, of least within The constraints imposed by The detection limits. An <br />exception is nitrate. While remaining well below the recommended criterion, the <br />mean concentration of nitrate increases significantly at the 5 percent level below <br />station TRA. As discussed previously, the common species of calcium, sodium, <br />magnesium and sulfate show increases below TRA and these increases are <br />statistically significant at the 5 percent level. <br />Revised 7-81 <br />2.5-42 <br />