A researcher is interested in determining whether taking additional vitamin C helps prevent the common cold. A randomised experiment was conducted to address this question. The study randomly allocated 279 people to either a group where vitamin C supplements were given, or a group where a placebo pill was given. These people were monitored and the numbers of those who got or did not get a cold were recorded. The results are summarised below:

Got a cold Did not get a cold Vitamin C 17 122 Placebo 31 109