T-test for two proportions

[mattc]

From each of 200 randomly selected grocery stores across the U.S., a Brand A broiler chicken was purchased and from each of another 200
randomly selected stores, a Brand B broiler chicken was purchased. All 400 chickens were tested for the presence of Salmonella. It was found
that 14 Brand A chickens and 20 of the Brand B chickens tested positive for salmonella.

Find the P-value for a statistical test of whether or not the incidence rate of salmonella is the same for both brands. What do you conclude?

[omega]

For the t-test for 2 roportions, the test statistic:
Z0 = (p1hat-p2hat) / sqrt[p1hat*(1-p1hat)/n1 + p2hat*(1-p2hat)/n2]
=(0.07-0.1)/sqrt[0.07*0.93/200 + 0.1*0.9/200]
=-1.08

Then you can calculate the two-tailed p-value.