Before you decide to use a select menu for a field ask yourself a question. Will users know their input without looking at the list of options? If the answer is yes, do not use a select menu for it. Instead, use an autocomplete field.
It’s unnecessary for users to scan and scroll through a long list of options if they already know their input. Select menus force users to do this extra work which frustrates them and slows them down. It’s even worse for them on mobile where only a small part of the list is visible and keyboard search is not available.
An autocomplete field is better because it saves users time and effort when they’re selecting known input. The user only needs to type the first few characters of their input before they see it appear as an option. They can then press the down arrow key and to select it from the menu.
Instead selecting from a large amount of options, they’re selecting from a minimal amount of options. The more they type the less options appear, making it even quicker to find and select their option. The number of options displayed to the user is reduced, which also reduces the error rate.
This usability violation of select menus is often found on fields involving time or place input. Country and state fields are big culprits as well as day and month fields. Users know their input for these fields and shouldn’t have to waste time scanning irrelevant options.
Make it quick and easy for users to fill out your form by using autocomplete fields instead of select menus for known input. It’s much like going to a familiar restaurant. If you already know what food you want to order, reading the menu is a waste time.