Arithmetic word problems with an unknown reference set, such as «John has 7 eggs. He has 4 eggs fewer [more] than Peter. How many eggs does Peter have?» are considerably more difficult for children than problems with an unknown compare set (second sentence: «Peter has 4 eggs more [fewer] than John.») Six experiments with 1st graders and kindergartners investigated reasons for this finding. Experiments 1-4 revealed that neither difficulties in processing the personal pronoun nor the use of key word strategies could explain the difficulty differences. Experiment 5 and 6 indicated that the lack of access to flexible language use is what makes compare problems with an unknown reference set so difficult.
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Elsbeth Stern (1993) studied this question.