Hiding Search Facets is a Bad Idea

While reviewing a site last week, I noticed the following behaviour in a faceted search UI:

1. Search for something as free text (eg “cups”); get a big list of cups and related items.
2. Use filters to narrow down the list by selecting a search facet (eg “plastic”)
3. Select another facet (eg “colour”).
4. I then see there is no facet for “red”.  There is only green, black and pink.

At stage 4, the facet for “red” is removed from view. I assume they do in fact have red cups, just not red plastic cups. That’s a safe assumption given the domain I’m in: I know that cups can be all colours, and it’s very unlikely a retailer would never have red cups in their inventory.

But what if the domain is unfamiliar? Take the following variation of the above:

1. Search for “USB charger” – get a large number of results
2. Filter by price – no higher than £5
3. Get 10 chargers under £5. Buy the one with the best reviews.

However, a problem might have arisen while my attention was on the price filtering. This is because what I didn’t realise at step 3 was the retailer also has solar powered chargers. But either I didn’t know such things existed, or I did but concluded that the retailer didn’t stock them (I wasn’t looking for “solar powered” in step 2, so might not have noticed them among the other facets).

Had I known they had solar powered chargers, I might have re-evaluated my decision to filter by lowest price, perhaps paying extra for something cool that I’d never originally considered. My unfamiliarity with the domain of USB chargers meant hiding the facet did me a disservice. I might even see solar powered chargers listed on another retailer’s site and conclude in error that the the first retailer didn’t sell them.

How might this be affecting search behaviour? I see that Amazon exhibits this pattern, which I assume they will have tested. But is converstion worse if all facets are shown? This is a bit of a mystery.

So as a result of this thought experiment at least, I would suggest not hiding facets with zero results. Show them with zero counts, grey them out, or put them under a drop-down if you have to. And if you have too many to show, perhaps you shouldn’t have so many in the first place? In theory, facets are supposed to be mutually exclusive, after all.

At the very least, you need to base these decisions on the domain you’re in; how often people use the the facets, and if they do, how it affects your conversion/KPIs.

As an aside, I wonder if the phenomenon of hiding factets is related to the fear of “overwhelming” people? This fear comes up a lot when dealing with complex interface design. In fact, just about all junior visual designers I have ever worked with will immediately seek to hide interface elements from view to “simplify” the experience, rather than use their skills in graphic design to make interaction unecessary.