That is fascinating. I'm going to have to check other months, now, and see if I can find other ones like this.

It's a curve fit probably, but a very good one. It would be lovely to see automated, randomised choices of start and end dates, but a few particular choices of dates confirms the adage for as far back as Quantopian's data go, as far as I can see.

(The adage being "Sell in May and go away until St. Leger's day")

The only time there was a problem was in Oct 2008; if you ignore that and imagine you would have stopped yourself from buying into a hugely down-trending market, results are beyond-ridiculous good.

@Dan: there is plenty of arguments for other months, too, but it's end of April / May that curve-fits ^GSPC best: http://sanzprophet.blogspot.co.uk/2012/12/seasonals-sp500-euro.html

...and these papers have decent coverage of this type of seasonality: http://papers.ssrn.com/sol3/papers.cfm?abstract_id=2138547 and http://papers.ssrn.com/sol3/papers.cfm?abstract_id=2115197 .

It is an amazing little adage. An article in S&C this month examines it yet again:
http://www.traders.com/index.php/sac-magazine/current-contents/feature-articles/45-classic-techniques/2070-sell-in-may-and-go-away

It's a "go away", not "buy bonds" approach, but amazing how much holding power this one has.

I've heard the adage a million times and like the results. But I'm curious does any one have any ideas of what is logically causing it to happen and where the flow of money goes?

Who is taking money off the table in such a big way, could it be hedge fund traders heading out to the Hamptons? or is it really just an unexplainable phenomenon.

i always thought it might correspond to year-end bonus season. depending on inudstry/position people get bonuses dec thru may. a corporate balance holds far less stock and more near-cash like investments than a retirement account. bonus season transfers money from corporate balance sheets to retirement accounts, so there is a net positive demand for stocks.

as a caveat, i have done basically nothing to test this hypothesis. i have some ideas on how to, but have never gotten around to it.

@Eric - there is no universally accepted consensus on the underpinnings of this 'seasonality effect' - also sometimes called the "Halloween indicator" - anymore than there is a consensus on whether it's fact or fiction. The two most commonly cited candidates that I've come across are seasonal variations in turnover volume and in AUM.

(1) seasonality in trading volume - basically your Hamptons hypothesis (interesting academic paper from mid 00s on the idea investors are "gone fishin'" in summer months). This study did also make an attempt to back out retail vs. 'smart money' participants based on trade sizes to try to see what investor class specifically is backing off - but they didn't observe a marked difference there.

(2) seasonality in investor flows - as @Scott suggests above - money flows into mutual funds are higher in Q1 driving demand and pricing in 9 months of expected returns in the first 3 months of the year.

I'd also point out that this is by no means the only seasonal anomaly out there - other fun variants that could be mocked up in Quantopian rather easily are:

• Seasonal tax avoidance - harvesting capital losses in December drives selling pressure and pushes equity prices down
  
• January effect - reinvestment drives buying pressure and pushes equity prices up
  
• Turn of the month effect - institutional $ flows are higher at the start of month than end of month (micro version of 'sell in may') which drives outperformance in week 1 vs week 4 on a monthly basis.
  
• Monday effect - increased selling pressure on Mondays (attributable to news cycle or the Office Space 'case of the Monday's' depending on your view)
  
• window dressing effect - fund managers realign their portfolio in advance of transparency requirements so they're holding the prior quarter's winners - drives quarterly price momentum

Instead of taking data all the way back, use a 10 year seasonal time frame and step it forward. Also, use best weeks in a year instead of months. The results should be better. The reason behind a 10 year (You can justify other time frames) is that the market is dynamic and seasonal trends do shift.

@Anony Mole: why do you think the curve for Buy in May and Play in the Hay would be anything but terrible?

@Anony Mole - the dates were different. I ran it again with the same dates (and initial captial) as before; you can see the benchmark returns are (almost ?!) the same. Remember this strategy is invested at different times than "Sell in May..." and that as returns are re-invested (in both), there is no perfect symmetry to be observed in the return graph.

This is my favourite algo on this site. How would it stack up if it did a sell and walk away, instead of reinvest into bonds?

how do i add picture in reply ,
any way ..
May-Oct avg returns 1.15% ranks at the bottom ( 12/12) for any 6 months period.
May-Oct , number of up periods are 62% again ranks at the bottom ( 11/11 , along with Apr-Sep)
May-Ocy , if returns are positive averaged at 6.77% again ranks at the bottom ..
from http://paststat.com/blog/sell-in-may-and-go-away-data-torture/

@Daniel Henry - that's my first reply

@Pallas, my mistake. I see it now!

Also, I read somewhere that election years are more stimulating for the economy. I wonder how holding on to the S&P on election years would look.

Sorry for the double post, but I made an attempt at reworking the algorithm to cater to election years. :)

@Daniel, it's not entirely clear to me what logic you meant to express -- did you mean to only buy in October the year before an election year and sell in the May of the election year, or did you mean to buy in October and sell in may, but if the year is an election year don't change any positions (don't buy, don't sell, just stay in the market from the previous October's)?

The former doesn't do so well (see backtest). The latter, which it looks like is what you've implemented, seems to do quite well :)

@Daniel I tried to implement your "sell in may (but not if it's an election year) and buy in october", and got almost the same numbers as you...it seems very good.

@Daniel if you start in 2003 instead of 2002 you give the benchmark a head start of all that first three-quarters of 2004 rebound, and the benchmark actually wins. This is why real statistical analysis (see @Peter's paststat.com link) is going to be quicker than data-mining by running backtests :)

@Pallas, good observation. I backtested the algorithm again for every year from 2002-2011, and the results were as follows:

Year / Benchmark / Algorithm / Ratio
2003 / 97.09 / 79.84 / 0.822
2004 / 62.9 / 54.09 / 0.86
2005 / 55.88 / 54.09 / 0.968
2006 / 38.93 / 41.3 / 1.06
2007 / 20.64 / 24.92 / 1.207
2008 / 28.29 / 103.14 / 3.646
2009 / 84.29 / 103.14 / 1.224
2010 / 35.22 / 75.81 / 2.152
2011 / 17.39 / 46.29 / 2.662

It's hard to tell how an election would influence the algorithm, because the 2008 crash does wonders for a "Sell in May" type of investing. For that reason, I would consider 2008 as an outlier, and therefore, neglect its results. More data and tweaking would be needed before I would confidently infer anything. :)

edit: Though, omitting 2008, the data show a progression in the ratio (i.e. the algorithm seems to be increasing every year).

I'm not sure how to parse images, so here's a link to a graph:

http://i.imgur.com/LMXTNRm.jpg

Also, could someone explain to me why the "Sell in May, and walk away" strategy is, for lack of a better term, crapped on by almost everyone, while historically (since the very beginning of the S&P) it has been shown to outperform the hold-all-year-long strategy? I must be missing something here...

@Daniel - The efficient market hypothesis says this trend shouldn't exist, since it's quite predictable.

Hi everyone,

Thought you would find this economic seasonality piece in Institutional Investor interesting and perhaps relevant: http://www.institutionalinvestor.com/Article/3221085/Reading-Between-the-Lines-of-Seasonal-Economic-Data.html

thanks,
Jean

if you dont sell in may.. shud that be equivalent to by and hold..?? looks like.. and hold bets sell in may... with $1,000 initial capital ;)

I have a hunch your leverage may be out of control.

I just clone it from jessica and instead of bond.. buy back the stock which make it techniklly.. buy and hold... just cloning performance sell in may code source code...

@Spencer... just buy and hold apple... Total Returns 6067451648.8%... Max Draw down only 354.8% only.. $1000 initial capital.

I have a friend that works for an investment firm and he said that their analysis showed that the 3rd year of a president's term exhibited market uptrend. His hypothesis , the candidate was doing everything possible to show that current administrative policies have had positives impact on current market conditions. However, no bets, on second term elections. Hope this helps.

J.D. Grey