Listed below are amounts of strontium90 in millibecquerels o

Listed below are amounts of strontium-90 (in millibecquerels, or mBq, per gram of calcium) in a simple random sample of baby teeth obtained from Pennsylvania residents and New York residents born after 1979 (based on data from “An Unexpected Rise in Strontium-90 in U.S. Deciduous Teeth in the 1990s,”) by Mangano, et al., Science of the Total Environment). Use a 0.05 significance level to test the claim that the median amount of strontium-90 from Pennsylvania residents is the same as the median from New York residents. Use the Wilcoxon rank-sum test.

Pennsylvania:

155

142

149

130

151

163

151

142

156

133

138

161

New York:

133

140

142

131

134

129

128

140

140

140

137

143

Solution

Correct Answer: Option (D)

high rank = 194.5 = W

mean = n1(n1+n2+1) /2 = 150

Variance = n1n2(n1+n2+1) / 12 =300

Z= (194.5 - 150) / sqrt(300) = 2.57

Critical z:        ±1.96

P-value = P(z<-2.57) + P(z>2.57) = 0.0051+0.0051 = 0.0102

Since p-value is less than 0.05, reject the null hypothesis that the populations have the same median.