The purpose of this research was to find a problem through lesson planning and to examine whether students understand mathematical probability accurately, conducting the lesson and analyzing the result of the lesson. This research focused on the notion of "Equally Possible" in a unit "Probability" in the second year of lower secondary school. "Equally Possible" was the essential notion for considering mathematical probability.
The result revealed that many students considered and analyzed mathematical probability without consideration of "Equally Possible" The cause of the findings was considered as lack of experience in two things: challenging the assumption and making an assumption to consider the event. Additional research with planning the lesson to solve the problem and to verifying the effect is needed.