Question 2 (10 pts) A Swedish study examined gingival bleeding in 164 pairs of same-sex twins of which one twin had low exposure to smoking, while the other had high exposure to smoking. The reasons for using this co-twin control design include pair matching for sex, age, and genetic factors, and that confounding factors such as oral hygiene and food habits are also likely to be matched within a pair. The aim of the study is to test whether smoking is associated with higher risk of gingivitis. The study found that in 40 pairs the low-exposed twin had gingivitis while the high-exposed twin did not, in 23 pairs the high-exposed twin had gingivitis while the low-exposed twin did not, while in the remaining 101 pairs the twins had same status: in 15 pairs both had gingivitis and in 86 pairs both did not have gingivitis.

1. Perform a statistical test for the null hypothesis, using SPSS. Interpret your results.