Is it common to cite conference papers in a PhD?

I tried searching for a similar question but most I read dealt with published papers and not a PhD, so apologies if this is a duplicate. I will remove this question if it is.

I've read a number of PhD dissertations and I haven't come across one which cites a conference paper, only journals (and the occasional website). I guess also it depends on the university if they accept such citations but I was just wondering if this is commonplace?