The scientists developed a new model to predict the height of Japanese cedar trees based on their diameter. They used a method that divided the trees in a stand into upper and lower groups. By analyzing the relationship between tree height and diameter, they created a more accurate model than previous ones. The new model had an error rate of around 6.8%, which was better than before. Even if there was a small error in measuring tree height, it didn't affect the model much. This new model can help estimate the height-diameter relationship in forests more effectively.