Question

An import threshold for mercury in crayfish is 9 parts per billion (ppb). A study obtained a random sample of 120 crayfish from a large lake. Of these, 24 have levels of mercury that exceeded 9 ppb. Past research had shown that 25% of the crayfish had levels that exceed 9 ppb. The researchers wanted to see if there was a change in contamination in crayfish. What is the test statistic for this problem? (use 2 decimal places)?

1.26

-.31

-1.26

-1.37

Answer 1

We have for given example,                      
Population proportion value is =0.25                  
x=24                  
                      
n=120                  
              
Estimate for sample proportion
=p= 0.2

                      
Z test statistic formula for proportion                      

2P-p


0.2 - 0.25 0.25*(1-0.25) 120 V

=-1.26      

The test statistic for this problem = -1.26