it would certainly seem less able candidates are taking it but, unless it has been badly taught in their schools, it is not an easy exam to do very well in as, with the uptake widened, the percentage getting Astar is a third of what it was previously.
I don't see your logic. The question is "what results would the same cohort have achieved had they taken the GCSE?" That's unknowable. The fact is that in the past couple of years, there has been a six-fold increase in the number of people taking the iGCSE in English, and that increase has come almost exclusively from schools at the lower end of the attainment spectrum. Why would such schools do that if they were harder? If they're targeting C, every D is an absolute disaster: these are schools that are already flirting with the floor standards. They must believe they'll get better results, as the cost of them getting worse results could be special measures.
with the uptake widened, the percentage getting A is a third of what it was previously.*
What do you deduce from that? Perhaps 50000 extra candidates who would otherwise have got D at GCSE have switched to iGCSE and got C. How would you rebut that hypothesis?
An exam is taken by a selective indie. The following year it's taken by the same indie, and a comprehensive school next door that is in an Ofsted category. The rate of Astar drops. What can you deduced from that? Very little.
perhaps the lower sets benefit from IGCSE as children with dyslexia and so on will find the question style easier to understand?
Why won't a simpler question style benefit all candidates?
I may be naive but I don't think some very academic schools would continue to use them if they did n't feel they were challenging enough as that would make the transition to A levels harder and impact on their results.
Again, I don't see how you can argue that. They're highly selective schools. They'll be expecting a majority of the A Level entries to result in Astar or A. For every very successful school using the iGCSEs, there will be at least one similarly successful school that doesn't, or doesn't in any quantity. happygardening is creating a false dichotomy: the most parsimonious interpretation of the data would be that the choice of GCSE syllabus is not a wildly important part of the success of schools at 18.