This classic question was researched by Tversky, Kahneman and Bar-Hillel;

"A certain town is served by two hospitals. In the larger hospital about 45 babies are born each day, and in the smaller hospital about 15 babies are born each day. As you know, about 50 percent of all babies are girls. The exact percentage of baby girls, however, varies from day to day. Sometimes it may be higher than 50 percent, sometimes lower. For a period of one year, each hospital recorded the days on which more than 60 percent of the babies born were girls."

Do you predict that;

(a) The larger clinic recorded more such days.

(b) The smaller clinic recorded more such days

(c) It makes no difference - the expected value is the same for both clinics.