1. The volume of land is relative to the volume of the market at a high level. Through the statistics of the trading volume data of the stock index at a high and low level in history, we can find that the standard of the volume of land has traces to follow.
 2. The standard to measure whether the intermediate decline market bottoms out is that the trading volume at the bottom should be reduced to within 20% of the highest trading volume at the top.
 3. If the trading volume is greater than this ratio, the stock index still has room for decline; On the contrary, the bottom is expected.
 4. For example, the famous "5.19" market saw the most obvious contraction, and then the bottom was built, creating a 17 month bull market.
 5. The same rule can also be verified by the degree of contraction of the bottom and the duration of the bull market in other times.