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Filters are applied right when the user adds or changes them (using ajaxAJAX). We decided to do this because it gives the user immediate feedback. During paper prototyping, we considered using a standard form with a search button or having a filter button for each category, but testing made it apparent that immediate ajax AJAX response was the best option and made the most sense to most users. One design decision that we considered was making a full text search available so that if a user wanted to search by poster or by location, the user would have the control and freedom to do so. We decided against this because we didn't want the search panel to be too cluttered, though in retrospect this design feature may have added more benefit than we realized.
When filters are applied, the user is shown a message that contains the tags they are searching for (as in the above image). This makes it clearly visible what is being shown and the user can easily clear all search filters by using the standard 'x'.
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