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Have the advanced search to update results instantly, when a user enters a search term or when they click the CLEAR button.
Therefore, remove the blue APPLY button.
Provide explanations for some FIELDS. For example:
Define the fields “Consists of,” “Has met,” “Is part of,” etc.
Describe the difference between fields like Date, Year, Creation date, Issued date, Temporal, etc.
Update the UI design so the user can more easily see the main search bar, the advanced search, and the filters at once. Most of the issue here is with the main search bar, and how it covers the advanced search and also disappears easily.
Also, make it more clear that typing a search term into the main search bar is the same as searching for “Any field contains”. This was not obvious to the user.
Test with a couple more people.