Why do some segments have keywords and other segments have none?

Keywords are attached to one-minute segments when a topic is discussed or described in some detail. If the discussion or description spans several segments, the relevant keywords are usually applied once. Keywords may appear at the beginning of the discussion, in the middle of the discussion, or towards the end of a discussion. New keywords appear when the topic of conversation changes. Please note: Approximately 4,900 testimonies within this collection were indexed using larger segments than one minute. For these testimonies, the actual segment length can range from one minute to nine minutes. Each segment in these testimonies usually has multiple keywords.