Say the site is for searching cars for sale, one of the filters is "only show black cars", if there's a car listed which doesn't have a color specified, should it be included in the search results or not?
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Some sites have "near miss" matches after their exact matches.
If a user has exhausted everything precisely matching their criteria but is still looking, there's a clear line and an explanation that this is the end of the results, then "near misses" sorted by how close to matching they are.
"Unspecified" would rank higher in this formula than "specified and wrong".
For example, it might look like this:
It's a good compromise because:
If there were no matches at all, you'd want a VERY large, prominent "Sorry, no results" box, around the size of those matches in the mockup above, so people don't see the (potentially relevant, potentially irrelevant) results before realising they're suggestions not matches, and then thinking something is broken.
No. For the user to actively specify a color, then see colors which do not match their specification risks confusion and a lack of confidence in the accuracy of the tool.
An alternative would be to add a sub-category of "Unspecified".
If you want to do something really interesting, crowd-source the data in the Unspecified category and ask users to say what color the car is.
You can mitigate the issue by:
Displaying this category of cars at the bottom of your search results so cars that do meet the full search criteria are presented first (higher relevance), only caveat here is to clearly emphasise visually and via adequate labelling the results that do not match the colour criteria but tick all other requirements.
Hiding the colour option within a collapsable filter
It is perfectly acceptable to collapse filters to just a label, providing a single link like View All Filters Source: Best Practices for Designing Faceted Search Filters
This will increase search accuracy and help conversions for the majority of users for whom colour is not the main focus. (approach used by Auto Trader)
Overall not a good idea, consider the following (assuming that product count is dynamicly updated as in the example of auto trader):
When there is only limited number of cars with unspecified colour it would be a waste of precious page real-estate while also adding complexity to filtering options.
When there is a large number of cars with unspecified attributes "colour or otherwise" then an unspecified option erodes user confidence in the site and indirectly suggests that your customers don't provide full details about what they are selling.
The best approach is to present your results with clarity using clear and understandable filtering options and ideally:
“At every step in the search workflow, any visible filtering options should reflect only the inventory that is available.”
Instead of burdining your filtering options with an ambiguous option, rely on users to make a judgment call if they would like to contact the vendor if the car ticks all their other requirements.
Overall, the unspecified or other option is an anomaly because "category labels should accurately describe the information in the category" to help users anticipate what they will find.
This might be the result of technical constraints as you have highlighted or because of difficulties matching the search criteria, but the user should not be asked to do guess work as a result:
I am building an aggregator that pulls listings from a number of sites, some which have missing data (or don't enforce the data as a required field)
To conclude, filtering options are not only a search facility but also a navigational structure and labelling and navigation best practices should apply.