RSI - the most predictive ratio of all?

Monday, Oct 29 2012 by
RSI  the most predictive ratio of all

Stock pickers love ratios, but which is the most predictive ratio of all? Value investors often have a bias towards low PE ratios or high dividend yields, growth investors look for high EPS growth while those that prefer a blend often lean heavily on the so-called PEG. All of these ratios have some anchoring in one of the fundamentals of the underlying business - the earnings or dividends of the company. It seems smart to ensure that you are basing investment decisions on business fundamentals but the irony is that the most predictive ratio for future price returns is completely unrelated to the underlying business. It's based entirely on the recent movements in the company's share price and has come to be known as the Relative Strength Index or RSI.

The Relative Strength Index (RSI) was first developed by J Welles Wilder in a 1978 book called New Concepts in Technical Trading Systems. It is not to be confused with the better known plain vanilla Relative Strength which simply calculates the absolute price return of a share (using the starting and end prices over a time period) and compares it to the return on the market index. RSI is a little more complicated to calculate which is perhaps why it has traditionally been used more often by technical analysts and chartists than fundamentally oriented stock pickers. We think this is a shame as recent evidence shows that it may be much more predictive than plain vanilla Relative Strength.

How to calculate RSI

RSI uses all the price changes that occur over a given period in order to derive the average price change. Essential to the calculation is creating the ratio U/D calculated by dividing the moving average of the total price points gained on 'up' days (U) by the moving average of the total price points lost on 'down' days (D). This ratio is then 'indexed' to create a number that moves between zero and 100 as follows:

RSI = 100 - ( 100 / (1 + U/D) )

The reason technicians enjoy the RSI so much is that it can be used as an oscillator and graphically displayed underneath stock charts as depicted in the image below. Wilder believed that readings above 70 signalled an overbought stock whereas readings under 30 signalled an oversold one. RSI has generated a big following amongst day traders who use it over short time periods,…

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7 Comments on this Article show/hide all

marben100 29th Oct '12 1 of 7


Could you clarify, please: when you say "The top quintile of stocks based on their 28 week RSI", do you mean those with highest RSI reading (i.e. considered "overbought" in conventional T.A.) or the lowest RSI (i.e. most oversold).



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peterg 29th Oct '12 2 of 7

He means those with the highest RSI.

The trouble with this sort of analysis is that when you start looking into it in detail you start to find that the timespan of the RSI makes a big difference to the outcomes. Note in the article that the optimum timespan for longs and shorts differed. It's very easy to come up with spectacular returns looking at historical data - sadly carrying those returns forward is a hell of a lot harder!


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Edward Croft 29th Oct '12 3 of 7

In reply to peterg, post #2

Hi Peter - the following is taken from the book - it clearly shows the optimum timespan.  You probably could use anywhere from 22 to 38 weeks with equivalent results. 

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Edward Croft 29th Oct '12 4 of 7

In reply to marben100, post #1

Mark - yes highest RSI reading - top quintile / top 20% of the market by RSI reading. Remember this is a longer term indicator than is usually seen on TA charts. Those kind of charts often use a 14 day RSI which is I believe more of a classic 'overbought/oversold' indicator (I should have made that clearer). This is 28 week with weekly data. It's more of an intermediate 6 month momentum indicator in this scenario.

Blog: Follow @edcroft on Twitter
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drainbolt 19th Sep '15 5 of 7


In your Oct 2012 article on the predictive qualities of the RSI, you said that you would be making the 28 week and 16 week RSI available in the Stock Reports and the Screener "in the coming weeks". They do not seem to be there yet. Is it still intended to offer these indicators for screening and assessment purposes?

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mgallear 21st Sep '15 6 of 7

Does this mean that the RSI is going to be made a screenable factor in the near future?

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jimmygee 28th Sep '15 7 of 7

I too would like to know when the 16 week and 28 week RSI will be added!

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About Edward Croft

Edward Croft

CEO at Stockopedia where I weave code, prose and investing strategies to help investors beat the stock markets. I've a background in the City and asset management but now am more interested in building great stock selection tools for the use of investors online.   Traditionally investors online have had very poor access to the best statistics, analytics and strategies for the stock market and our aim is to set that straight.  High Quality fundamental information has been prohibitively expensive in the past and often annoyingly dull. People these days don't just want to know the PE Ratio and look at a balance sheet. They expect a layer of interpretation over data, signal from noise and the ability to know at a glance whether a stock is worth investigating or not. All this is possible using great design and the insights gleaned from quantitative research.  Stockopedia is where we try to make it happen ! more »


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