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.

Extended data:

Classification of critical value (i.e. threshold value):

1, PS threshold: the threshold in PS is actually a black-and-white boundary value based on the brightness of the picture. The default value is 50% neutral gray, that is, 128, and the brightness is higher than 128 (

2.AE threshold: the threshold can be understood as the value range, that is, the value range of the dependent variable. For example, in after effects, the threshold of a layer's transparency map is 0- 100. When the input signal is lower than the threshold, the gain will be enlarged or reduced according to a certain compression ratio.

3. Absolute threshold: Only when the stimulus reaches a certain intensity can it arouse people's feelings. The minimum stimulus that can just cause sensation is called absolute sensation threshold.

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