Stocks that have lost value should also be sold according to pre‑set rules — don’t try to even out costs by buying more

No cost at all. The hard part is admitting the loss and actually selling the stock.

This study examined every trade made by 10,000 accounts at a U.S. discount broker from 1987 to 1993. The proportion of profitable stocks sold (PGR) was 0.148, while the proportion of losing stocks sold (PLR) was 0.098 — a difference of 0.050 with a t‑value exceeding 35. In other words, rising stocks were sold more than 50% more often than falling ones. Investors also bought losing stocks more frequently: the re‑purchase rate for losers was 0.135 versus 0.094 for winners, again with a strong statistical significance. The data also contradicted the belief that “stuck stocks will eventually bounce back.” Over the next year, the excess returns of sold profitable stocks exceeded those of kept losing stocks by 3.4 percentage points. These excess returns represent performance above the market index. As a concrete illustration, selling $1,000 worth of a losing stock instead of a winning one would yield roughly 4.4% more return in the following year; this figure includes tax savings from the early sale under U.S. tax rules (1987‑1993).

Odean T (1998). Are Investors Reluctant to Realize Their Losses? The Journal of Finance 53(5):1775-1798. https://doi.org/10.1111/0022-1082.00072

This article governs 'whether to sell,' while Article 15 (not frequent trading of stocks) governs 'don't keep acting'; the two are not in conflict. Set the rules in advance and then execute accordingly; the author's experience is that the number of actions is actually fewer, though this point has not been measured by research. Averaging down costs by adding to positions later is arithmetically correct; buying a bit more makes the average cost slightly lower. The error lies in treating it as a way to turn things around. Its actual effect is that it increases the amount of money you have bet on this single stock; see Article 19 (don't bet all your money on one stock). The section on tax deductions is calculated according to the US tax system and does not apply directly to China, but the remaining parts still hold. This is account data from the US in the 1980s and 1990s; the trading costs and tax systems for A-shares are different. This does not constitute investment advice.