This study investigated the status of learning statistical inference based on the new high school curriculum guidelines, focusing on problems related to the standard normal distribution to assess understanding. Furthermore, based on past incorrect answer data for the standard normal distribution, effective teaching methods were proposed, implemented in educational practice, and their effectiveness was verified. Regarding problems on the standard normal distribution, having students draw graphs to understand the relationship with the formula and then review their work led to an increase in the correct answer rate. However, a significant number of errors persisted, including those based solely on rote understanding and careless mistakes. Measures to address careless mistakes still have room for improvement.