At what option count should I add a search filter to a drop down element?

Question

I have a booking tool that offers a list of services to choose from. The option list is variable per client. Some clients offer one service, some have twenty or more. One client requested a search filter on the drop down. We are honoring the request and it will go out to all users.

However, I don't want to add a the filter unless it's useful.

So, at what option count does the search filter become meaningful? I am setting that number as a constant and if the option count reaches it, the search input appears. I chose a number that is A. less than the clients service count of course :) and B. What I think is the right count but I wonder if my choice is arbitrary.

What is your number and why?

Answer 1

I think the number of items is not the crucial parameter. First, you should decide whether the filter is appropriate to the interaction.

The decision could be based oh two important parameters, which are User and Data related. This two dimentions are presented on the picture:

User-related

Filter is the mean to make selection faster. Users, which are competent in subject field, know what to search and filter allows to perform selection task in a faster way. Non-competent user doesn't know what to search and his interaction looks like exploring and then selecting.

Data-related

Second dimension is the nature of data. If the data if highly structured, user could perform selection task in a fast way, even without any support. Under structure I mean ordering, sorting, or some other organization of the data. This organization helps to navigate and select required item.

Non-structured data requires support to make selection task faster, and filter could be such supporting tool.

Examples

1. Competent user, Structured data (visual support by country's flag and language) – no filter.
   
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2. Competent user, Structured data (natural ordering) – no filter.
   
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3. Competent user, Weak-structured data (different type of data: city, district, region) – filter needed.
   
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4. Non-competent user (in exact category naming), Less-structured data (alphabetical ordering adds value) – no filter.
   
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5. Too many items – do not use dropdown!
   
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Answer 2

The rule of thumb we use at my workplace is if the dropdown has a scrollbar, provide a filter. (Of course, that requires that you have a number for when to use a scrollbar.)
Generally, also any dropdowns that are populated from lists that the user can add to also get filtering options. In many cases (in our system), it's as simple as switching from a dropdown control to a combobox control.

If the filtering control doesn't detract from the design, are there any pressing reasons not to use it?

Answer 3

A commenter here cites 12 items in a dropdown as the maximum one should use, but they're unable to provide any support for that number. If you assume that's the maxiumum number of items, then the search input would appear when the number of items exceeds that.

I would be inclined to think the number is contextually dependent on the implementation and verify its efficacy through testing. I'm not sold that there's a one-size-fits-all rule when it comes to this.