I remember two or three years ago when I often wondered why the FTSE index would jump up and down when none of the mainstream stocks had moved at all. It turned out that
Vodafone made up something like 15% of the market value of the top 100 companies. My gut reaction - not my training in business finance - told me that something was wrong.
Now, Vodafone has lost some two thirds of its former value. I'm sure it remains a good company but the "gut instinct" is often correct.