Sell in May / September Effect
SEPTEMBER EFFECT
Why does September have such a weak stock-market reputation?
September is often cited as one of the weakest calendar months for equities. The useful question is whether that tendency appears consistently in the index or stock you are researching.
How to study September seasonality
Do not rely on the month’s reputation alone. Test September as a recurring window, inspect every year and compare the result with August, October and the broader May–October period.
Monthly return
Measure the September return across many years, then compare average and median results.
Consistency
Check win rate and the distribution of positive and negative years instead of focusing on one crisis period.
| Comparison | Research question |
|---|---|
| September vs August | Is weakness unique to September or part of late-summer softness? |
| September vs October | Does performance recover after the month ends? |
| S&P 500 vs DAX | Is the effect market-specific? |
Test September on any supported ticker
Select September 1 through September 30, then review historical returns and yearly paths.