In my opinion, you can diversify at these levels:
Local (by headquarters, by turnover)
By currency
By industry
By company size (influences volatility)
I try to have as little overlap as possible in several areas in order to keep my portfolio robust.
Example:
$DRO (-0,11%) and $PARRO (-2,21%) : Similar industry, so both are driven by the same news, but have different locations and currencies.
$CACI (+0,05%) Also has some correlation with the two, but is mainly dependent on the movements of the US military.
$8001 (-1,36%) As a boring anchor
$SL (+0,08%) As a "real" luxury play to profit from rising inequality and to have more euro/Italy in the portfolio.
$GRE (-0,48%) Because I see great potential in Greece, see old post
