I tell them any time you use information from another source you need to say the source even if you use the same source 2-3 sentences in a row. Every time I hear you give a statistic I ask myself, "Did you give a source for that?" We do exercises where I give them paragraphs without citations and ask them to identify all the ones that need citations. I have them do impromptu speeches on summarizing an article and citing it multiple times. I explain the difference between a source and a citation. I tell them I don't care about a works cited slide. If I don't hear them say the source it doesn't count.
Many of them still give statistics without citing them.