Let's take an example: what is the correlation between bonds and equities? This question is the basis of asset allocation. The more negative is the correlation, the more likely are bond prices to rise if shares fall. This would mean bonds are a hedge against equities, which would mean we could hold lots of shares in the knowledge that we have insurance against them falling - just as we can drive faster if we have car insurance. A positive correlation, on the other hand, would mean that shares are riskier, as we have no such hedge.
So which is it? We can make a precise statement: Since January 1986 the correlation between monthly returns on the All-Share index and on the FTSE UK all government bonds index has been 0.15.