It's 'certainly', when I have my moments of doubt!
It used to be 'frequent loose stools' too before I learnt that 'diarrhoea' is 'Doesn't It Always Run Really Horribly Over Each Ankle'.
There is usually a suitable alternative if you are unsure of a spelling.
Oh and 'There, Their and They're', they all begin with THE, so no excuse for Thier. 'Here and there' for There, They are contracts to They're, so that leaves only Their for belonging to them!