Here's a better definition of trading stocks:
The purchase or short sale of shares with the intent of profiting on either upward or downward price movement, based off any of the following:
a) Technical information (market data such as price, volatility, volume, etc).
b) Fundamental information (company financial information)
c) News
d) Premonition
e) Completely random selection
f) Literally anything else, including a combination of the aforementioned.
The vast majority of people who trade completely neglect the concepts of position sizing, exit strategies, or other components of a solid trading methodology. In my opinion, this is due to an unhealthy industry obsession with entry signals.
Entry signals determine when you enter into a position, and that's it. They don't tell you how to stop losing money when the stock moves against you, and they don't tell you what quantity of stock to trade in the first place so that you don't expose yourself to undue risk.
It is possible to construct a profitable trading system that utilizes completely random entry signals. You won't make a lot of money, but most people are surprised by the very notion that something like this is even possible.
For example, say you randomly select 100 stocks to trade, and you allocate 1% of your equity to be risked per position. Your exit signals, assuming they're properly designed, will terminate a position once its losses have reached 1% of your total equity. Conversely, these exit signals will also allow for favorable price movement. In other words, they cut your losses and let your profits run.
If you replaced random stock selection with quality entry signals, the trading system I just described would be even better. Point being, even if you have the sexiest, most profitable stock selection method/entry signals in the world, but you fail to give thought to the other parts of your trading methodology/system, it's very possible, if not probable, that it will fail miserably.
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Psychology is an absolutely huge component of trading. The best traders, whether they're automated systems traders or discretionary traders, overwhelmingly tend to be utterly emotionally detached from the design and execution of their chosen system or methodology, as well as the outcome. Elation from profits can be just as dangerous as sorrow from losses. Exercising this kind of discipline is incredibly hard in practice.
Top traders also tend to be fiercely independent in regards to the synthesis of the ideas and opinions they hold concerning the market; there's a reason very few people get rich trading on the advice of newsletters.
I tend to agree with the article's premise though, in that that most people are probably better off not trading. Ultimately, markets are wealth transfer mechanisms that tend to concentrate wealth into the hands of the advantaged and competent. If this were anything but, things like high-frequency trading and private firms with vast profits wouldn't exist, or at least not to the degree they do today.
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As an aside, if anyone has an interest in trading and hasn't read them, I highly recommend Market Wizards: Interviews with Top Traders and The New Market Wizards: Conversations with America's Top Traders, by Jack D. Schwager. They were published in 1993 and 1994, respectively. Although written in a period where automated systems trading was in its infancy (HFT didn't even exist), their value in terms of trading psychology remains intact. Arguably they're even more interesting today, considering they're now period pieces.