All the gaps need to be filled: Marco Simioni

It's an old Wall Street saying--"All the gaps need to be filled"--and is widely known among traders both for its accuracy and its occasional fallacy. It holds true for many market indexes and many stocks. I'm primarily focusing on the S&P 500 etf: SPY. Gaps in SPX are not as clear as in the SPY graph, which is why I will be referring to the SPY.

Consider up gaps to occur when today's opening price is higher than yesterday's high; and, consider down gaps to be those with today's open lower than yesterday's low. It is clear how all down gaps have been closed by the market so far. Most of the gaps take just a few days to be covered, but some take years. Like big holes or magnets they attract prices back to them. Let's talk first about unfilled down gaps.

Unfilled down gaps have shown the most consistent behaviour. All of them have been closed by next markets' bullish moves. A bull market can take years to recover and close a gap, however, all bear markets have resumed to new all time highs. This is mainly due to the general upside bias of the economy.

The main problem is not if a down gap will be closed, but WHEN. Entering big bear markets like 1929, 2001 or 2008 too early results in huge losses because the down gap can take a lot of time, potentially years, to be filled. Past stock market behaviour has shown down gaps filling 100% of times and this must be considered as a strong potential warning with short trades during even the -50% or more bear markets. The down trend will resume to cover all unfilled down gaps. Concerning long side investments, down gap filling acts as an insurance: waiting for the highest down gap to be filled is neither a waste of money nor of time.

Moving on to unfilled up gaps, different results arise from their analysis.

I collected all the unfilled up gaps in the table below going back to SPY inception in 1993. The first unfilled up gap occurred on February the 3rd 1995. And since that time, it is still not covered. We can count 20 other unfilled gaps from then to the present. And we can count about 20 years since the first of them occurred. Filling in the lowest unfilled up gap, as it appears from the backtest below, would mean the SPY would have to lose more than 75% from today's close at 207,5 (time of writing). We could argue that neither 2001 nor 2008 bear markets managed to fill those gaps, and those 20 old gaps won't fill at all. Never.

Since SPY inception 268 up gaps were filled, with only 21 not filled. 92,73% of times the market acted as expected to. Markets act with symmetry, and I believe these gaps to be filled in order to equal 100% accuracy of down gaps; we can't know the time, but they will. In 1995 there were not only 1 but 6 unfilled gaps, and all of them pointing south more than 70%. I think about them as the most dangerous 6 holes in the Wall.

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The was a guest article written by Marco Simioni of Nightly Patterns. Many thanks to him for his insights on gaps in the SPY.