Posted by Erik on .
Lead (Pb) concentrations were measured in waterfowl eggshell samples from two locations. In n1 = 41 samples from a location downstream from a paper mill, the mean Pb concentration was 1 x = 0.13 ppm with a standard deviation of s1 = 0.04 ppm. In n2 = 31 samples from a location upstream from the paper mill, the PB concentration was 2 x = 0.09 ppm with a standard deviation of s2 = 0.05 ppm. Test the hypothesis of no difference in mean Pb eggshell concentrations versus the alternative that the locations downstream from the papermill have a higher mean Pb concentration. Use a 1% level of significance.

Statistics 
PsyDAG,
Z = (mean1  mean2)/standard error (SE) of difference between means
SEdiff = √(SEmean1^2 + SEmean2^2)
SEm = SD/√n
If only one SD is provided, you can use just that to determine SEdiff.
Find table in the back of your statistics text labeled something like "areas under normal distribution" to find the proportion related to the Z score.