Average Difficulty Point

(or 'ADP' for short) is the summation of all the stage levels within a challenge, divided by the total number of stages in that challenge.

Generally, the higher the ADP, the more difficult it is.

Calculations
The ADP of Challenge 1 is 1.4. The calculations are as follows:

There are 10 stages in Challenge 1 with the following stage levels: 0, 1, 1, 1, 1, 2, 2, 2, 2, 2

The sum of these numbers added up to 14. This number is divided by 10 (the total number of stages in Challenge 1), and you get 1.4.

Limitations
ADP may not be an accurate difficulty indicator. Supposed a challenge has a lot of Lv2 stages, and one Lv8 stage. The ADP will be stated at a lower value.