An update on the Sell in May Effect (also called the Six-Month Effect, or Halloween Effect in the US).
In the six months Nov 2016 to Apr 2017 (Winter period) the FTSE All-Share Index rose 5.2%. Previously, the Index had risen 10.1% over May 2016 to Oct 2016 (Summer period).
The out-performance of the Winter market over the Summer market was therefore -4.9 percentage points, which does not support the Sell in May Effect.
The following chart shows the out-performance of the FTSE All-Share Index in the Winter period over the previous Summer period since 1982.
In the 17 years since 2000 the Winter market has outperformed the previous Summer market 11 times, with an average out-performance of 4.6 percentage points.
As can be seen in the above chart, while in the longer-term the Sell in May effect is strong, in recent years it has become less reliable.