If the index fund price is lower than the investor's cost price, then investors had better increase their positions. At this time, increasing their positions will reduce the cost of investors. The lower the cost, the less risk investors will take, and the greater the probability of recovering capital or gaining income in the future. If the index fund price is higher than the investor's cost price, then investors do not need to add positions. At this time, adding positions will increase costs, which is not cost-effective.

When investors suffer losses, they can set a goal of adding positions. For example, the goal of adding positions is 10%, and they start adding positions when they lose 10%. Adding positions is usually not done at one time. When the fund falls to 10% again, they will increase their positions again, and so on until the increase is completed, and then stop when the fund starts to rise.

The lower the cost price of the fund, the better. The lower the cost price, the lower the risk that investors take and the higher the probability of earning in the future. On the contrary, the higher the cost price, the greater the risk that investors take and the greater the probability of future losses.