Conversions are directly proportional to user experience so make sure your drop down menu is user friendly.
If all items are visible properly in drop down menu then it can increase usability of drop down menu.
All menu items should be neat and clean by using separate lines and alternate color combinations. Give some space between items for breathing. It will make it more beautiful and easy to understand for users.
Now the point is why we replace drop down menu with radio button/checkbox and what the criteria are for this. Radio button/checkbox items remain visible by default but Drop down items don’t visible before user click. So if drop down menu has less than 6 items then we can replace drop down with radio button/checkbox. It provides more usability and visibility to users. Check it out in an example – I have increased 22% growth in queries.
One more thing to improve user experience is auto suggestion in search box which is inside the drop down menu.
– Provide clear all option to users so they can easily refresh the drop down checkboxes
– Always show selected items on the top.
Fully highlight the dropdown menu when the cursor goes over it creating more visibility for users.