diff --git a/src/applications/search/management/PhabricatorSearchManagementNgramsWorkflow.php b/src/applications/search/management/PhabricatorSearchManagementNgramsWorkflow.php
--- a/src/applications/search/management/PhabricatorSearchManagementNgramsWorkflow.php
+++ b/src/applications/search/management/PhabricatorSearchManagementNgramsWorkflow.php
@@ -21,7 +21,7 @@
             'param' => 'threshold',
             'help' => pht(
               'Prune ngrams present in more than this fraction of '.
-              'documents.'),
+              'documents. Provide a value between 0.0 and 1.0.'),
           ),
         ));
   }