When putting references in written work - how should one order them? 
Alphabetically by first author, or chonologically by year?

(Barnett et al. 2005; Øvretveit et al. 1997; Watson 2004; )

or

(Øvretveit et al. 1997; Watson 2004; Barnett et al. 2005)

?

I am tending towards the chronological, but I am not sure if there is a 
preference.

All pointers appreciated!