I suspect synthetic phonics schemes have been successful because they do two things that whole-word or analytical methods do not.
First, they present information in a way that is explicit and uses sequential reasoning: "This grapheme represents this sound, which does this job in a word" - popularly described as "left-brain" processing.
Many children use holistic pattern-matching techniques - so-called "right brain" processing - in much of their learning, and appear to learn to read intuitively. In others, this ability is weak in some domains, and they need to have material presented explicitly: "This is how you hold your pencil; this is how you form this letter."
Whole-word methods pose problems for these children, but synthetic phonics schemes do not pose difficulties for holistic readers, and synthetic phonics is accessible to more children.
Synthetic phonics schemes often use a multi-sensory approach that associates a grapheme with a sound, a picture and a movement, reinforcing concepts for children whose sensory processing is still maturing and may be weak.
Of course, in reading a child uses a number of cognitive processes; if any one of these is impaired or immature, reading problems will result. Rather than compare one system with another, the Government should invest in research into cognitive processing in general. This could shed some light on our continuing failure to resolve problems with literacy and numeracy - and behaviour for that matter.
Market Drayton Shropshire