Final answer:
The variable "ui" in the formula represents the upper limit of the ith class interval.
Step-by-step explanation:
The variable "ui" in the formula x = a + h(σfiui) / σfi represents the upper limit of the ith class interval.
In a grouped frequency distribution, the data is organized into intervals or classes. The variable "ui" represents the upper limit of each class interval. By using the upper limits, we can calculate the mean of the grouped frequency distribution.