I came across this blog on PCPRO recently and thought it worth reproducing verbatim.

http://www.pcpro.co.uk/blogs/2011/02/09/how-one-small-design-change-can-kill-your-conversion-rate/

BigCommerce, a hosted eCommerce service, recently pushed out a minor update to all its shops including ours. Developers Interspire allow shop owners to schedule the upgrade to a convenient date and, as part of that process, require them to tick a box that says, in effect, "I understand that this may mess up my store design".

So, once the upgrade had taken place, I checked our shop to see what damage had been done to discover that the menu system had changed. Gone was the simple, tree-type list of categories and subcategories to be replaced by a swanky new "fly-out" menu which, as I'd made changes to the text colour in the main shop, featured black text on a dark background.

After half an hour's feverish work sifting through the various CSS rules to find out which controlled the text colour, I had a visible menu system again. So, for Tuesday and Wednesday of that week, we ran with the new menu. It was a disaster.

Over those two days, the conversion rate plummeted by 75%. I would hardly have believed it if I hadn't seen it for myself. By Wednesday evening I was in full panic mode. I finally found the checkbox for restoring the tree menu and, like flicking a switch, the conversion rate recovered instantly.

Why? I can only guess, because I'm not prepared to chuck money away experimenting with it, but I think there are at least two ways in which the fly-out menu is inferior. Firstly, if you have relatively few categories with lots of subcategories, the tree menu shows this to prospective customers more effectively: the shop looks as though it has a wide range of products. Secondly, the fly-out is more fiddly to use as it requires the user to hover over the main category name before clicking. In almost all cases with user interface design, simplest is best.

I'm angry that Interspire enabled the fly-out by default - it seems much more sensible to render the shop as it was and let owners know that a new menu type is available should they wish it trial it. But the big lesson is the effect that seemingly tiny changes to interface design can have on a business: it can mean the difference between a profitable enterprise and a basket case.