In the independence test, the critical value (also called threshold) is determined according to the actual results of the experiment. Only when the absolute value of correlation coefficient r is greater than the critical value can the relationship between two variables be approximately expressed by a straight line. Critical value refers to the condition that a physical quantity must meet when an object changes from one physical state to another, which is equivalent to stagnation point in mathematics.

In some cases, the critical value is equivalent to the maximum value, and we can get the maximum value through the critical value, thus getting twice the result with half the effort. In other cases, there is no necessary connection between the maximum value and the critical value. Therefore, in the process of finding the maximum value, we can only look for the critical point as a method of finding the maximum value, but not equate it with finding the maximum value.