Understanding how this interpretation of kelly criterion helps the trader
Three important things worth remembering about Kelly when applied to real world edges: 1) Full Kelly staking is gut wrenchingly volatile. While it maximises the growth of the bankroll, it does so in a way that still leaves you very likely to experience massive (50%+) reductions in capital. Most long terms users of Kelly tend to stick in the 1/4 to 1/2 Kelly unit range to try and stay sane and retain a margin of error. See below for how large the typical swings can be with full Kelly:  2) Garbage in, garbage out. If you are making errors in pricing your actual edge, Kelly becomes very wrong very fast, easily leading you to a high chance of ruin if you are over estimating your true edge. As most people do massively over estimate their edges, Kelly simply pushes them far into territory where risk of ruin is high. 3) A Kelly user prefers to back likely outcomes over non likely ones, even to the point where they prefer a smaller % edge if the chances of winning are better. Compare the below comparison of growth between two betting scenarios (decimal odds, so for the percantage chances do 1/odds):  In this case, despite the percentage edge on the red bet being higher than that of the green, in terms of bankroll growth it ends up only being roughly as good to a kelly gambler as the smaller edge on the more likely event. This has an obvious effect on the types of edges you should be seeking out if given choices between liklihoods. The goal of the kelly criterion strategy is to find a balance between preservation of starting capital and returns. One of extreme you could bet the entirety of your account on one trade, which would maximize your returns if you win, but leave you unable to further invest if you lose. On the other extreme, you could bet the smallest amount of capital possible over the course of several trades to increase the probability that you'll even out to 70% accuracy over time. But this method would be extremely slow.  So for your case, investing 40% each time is one way to find an optimal balance between these two extremes. Use this as a rule of thumb though, because your own situation and investing goals may differ from the goal of  optimal growth. I don't know too much about the kelly criterion, but going by the other answers it sounds like it could be quite risky depending how you use it. I have been taught the first thing you do in trading is protect your existing capital and any profits you have made, and for this reason I prefer and use Position Sizing (PS). The concept with PS is that you only risk a small % of your capital on every trade, usually not more than 1%, however if you want to be very aggressive then not more than 2%. I use 1% of my capital for every trade. So if you are trading with an account of $40,000 and your risk R on every trade is 1%, then R = $400. As an example, say you decide to buy a stock at $10 and you work out your initial stop to be at $9.50, then our maximum risk R of $400 is divided by the stop distance of $0.50 to get your PS = $400/$0.50 = 800 shares. If the price then drops after your purchase, your maximum loss (subject to no slippage) would be $400.  If the price moves up you would raise your stop until your potential loss becomes smaller and smaller and then becomes a gain once your stop moves above your initial purchase price. The aim is to make your gains be larger than your losses. So if your average loss is kept to 1R or less then you should aim to get your average gains to 2R, 3R or more. This would be considered a good trading system where you will make regular profits even with a win ratio of 50%.
"Below is just a little information on short selling from my small unique book ""The small stock trader"":  Short selling is an advanced stock trading tool with unique risks and rewards. It is primarily a short-term trading strategy of a technical nature, mostly done by small stock traders, market makers, and hedge funds. Most small stock traders mainly use short selling as a short-term speculation tool when they feel the stock price is a bit overvalued. Most long-term short positions are taken by fundamental-oriented long/short equity hedge funds that have identified some major weaknesses in the company. There a few things you should consider before shorting stocks: Despite all the mystique and blame surrounding short selling, especially during bear markets, I personally think regular short selling, not naked short selling, has a more positive impact on the stock market, as: Lastly, small stock traders should not expect to make significant profits by short selling, as even most of the great stock traders (Jesse Livermore, Bernard Baruch, Gerald Loeb, Nicolas Darvas, William O’Neil, and Steven Cohen,) have hardly made significant money from their shorts. it is safe to say that odds are stacked against short sellers. Over the last century or so, Western large caps have returned an annual average of between 8 and 10 percent while the returns of small caps have been slightly higher.  I hope the above little information from my small unique book was a little helpful! Mika (author of ""The small stock trader"")" I once saw Kelly on a Yahoo show and I had no idea who she was. After visiting this website, I now know that she is great personal stylist who is also a talented host. I was also impressed by this particular site on her website: http://www.kellylundberg.co/kelly-lundberg-show-reel/ &gt; It will have minimal effects on buy &amp; hold traders since they typically research for a long time, then buy &amp; hold stock for many months.  This is the part I never understand. If a short term tax doesn't affect buy &amp; hold traders, when why would HFT affect buy &amp; hold traders? "My study times were as follows:  * L1:  Started studying for the December test in mid-September; spent approximately 110 hours total studying Schwesser materials.  * L2:  Failed the first time w/120 hrs of studying from Schwesser.  2nd time around started in February (of the year of the test), spent about 130 hours studying straight from CFAI.  * L3:  Started in February of the test year again, approximately 130 hours of studying CFAI material.  I almost never studied more than 2 hours straight at a time, but it was consistently 2 hrs a day - that matters.  How much you study is up to you.  You need to study until you can confidently do the problems they're going to ask (they actually telegraph these questions directly to you if you study the CFAI materials).  The number of hours spent studying, outside of the construct of ""do you understand this and can you replicate it,"" is irrelevant.  Good luck -  PS I had approximately 8 years of front-office experience as a sell-side market maker and buy-side execution trader before starting L1.  I had absolutely no understanding of corporate finance, financial reporting/analysis, and very little understanding of equity valuation before embarking on the coursework." "This is because volatility is cumulative and with less time there is less cumulative volatility. The time value and option value are tied to the value of the underlying.  The value of the underlying (stock) is quite influenced by volatility, the possible price movement in a given span of time.  Thirty days of volatility has a much broader spread of values than two days, since each day benefits from the possible price change of the prior days.  So if a stock could move up to +/- 1% in a day, then compounded after 5 days it could be +5%, +0%, or -5%.  In other words, this is compounded volatility. Less time means far less volatility, which is geometric and not linear.  Less volatility lowers the value of the underlying.  See Black-Scholes for more technical discussion of this concept. A shorter timeframe until option expiration means there are fewer days of compounded volatility.  So the expected change in the underlying will decrease geometrically.  The odds are good that the price at T-5 days will be close to the price at T-0, much more so than the prices at T-30 or T-90. Additionally, the time value of an American option is the implicit put value (or implicit call).  While an ""American"" option lets you exercise prior to expiry (unlike a ""European"" option, exercised only at expiry), there's an implicit put option in a call (or an implicit call in a put option).  If you have an American call option of 60 days and it goes into the money at 30 days, you could exercise early.  By contract, that stock is yours if you pay for it (or, in a put, you can sell whenever you decide). In some cases, this may make sense (if you want an immediate payoff or you expect this is the best price situation), but you may prefer to watch the price.  If the price moves further, your gain when you use the call may be even better.  If the price goes back out of the money, then you benefited from an implicit put.  It's as though you exercised the option when it went in the money, then sold the stock and got back your cash when the stock went out of the money, even though no actual transaction took place and this is all just implicit. So the time value of an American option includes the implicit option to not use it early.  The value of the implicit option also decreases in a nonlinear fashion, since the value of the implicit option is subject to the same valuation principles. But the larger principle for both is the compounded volatility, which drops geometrically." It sounds to me like you may not be defining fundamental investing very well, which is why it may seem like it doesn't matter.  Fundamental investing means valuing a stock based on your estimate of its future profitability (and thus cash flows and dividends).  One way to do this is to look at the multiples you have described.  But multiples are inherently backward-looking so for firms with good growth prospects, they can be very poor estimates of future profitability. When you see a firm with ratios way out of whack with other firms, you can conclude that the market thinks that firm has a lot of future growth possibilities.  That's all. It could be that the market is overestimating that growth, but you would need more information in order to conclude that.  We call Warren Buffet a fundamental investor because he tends to think the market has made a mistake and overvalued many firms with crazy ratios.  That may be in many cases, but it doesn't necessarily mean those investors are not using fundamental analysis to come up with their valuations. Fundamental investing is still very much relevant and is probably the primary determinant of stock prices.  It's just that fundamental investing encompasses estimating things like future growth and innovation, which is a lot more than just looking at the ratios you have described. The early bird catches the worm. The first person who makes use of the information gains! That is why hedge funds pay billions of dollars to place their routers right at the center of wall street. Moreover, the information is not always correct. The article you are reading may be a rumor spread by someone on wall street.Then there is speculation and that is factored into the price. For example:- In spite of all the bad news from Greece, the market still continued to rise. This was because, everyone had an idea about what was going to happen and the price was factored in way before Greece actually defaulted. The game is way more complicated than it seems. If everyone sat down and read reports, opportunities to make millions of dollars would have been lost in those few seconds. (Please note:- I do not mean reading reports is bad) I remember like 20 years ago someone did an experiment where Peter Lynch, elementary school kids, and an ape, all picked stocks in a situation similar to this. The result was almost the same, with the elementary school kids coming out slightly better  Picking correct stocks is a craps shoot, as the market is an emotional entity, and in the short run, even educated guesses don't give an edge. People like Peter Lynch eventually win of course because of discipline, long term goals, and knowledge of tools like derivatives