Wide word spacing makes visual distinction between words easy for readers. This is especially important for struggling, new readers. Many characters reappear thruoght the PM books. Children relate to these familiar characters and look forward to reading about them. Fiction stories with science topics are scientifically accurate.
Children enjoy fiction stories that also teach science concepts.
The PM levels build on each other and high frequency words are revisited frequently. Teaching high frequency words in context is important. Using the PM Collection as a whole is more beneficial than using just certain levels.