GCSE grade boundaries

[Kensingtonia]

Sure this question has been asked before but as DD sat biology and chemistry core papers last week and found one tricky and one quite easy...how do grade boundaries work. Do you have to get a fixed percentage say 90% to get an A. Can anyone say what the boundaries are between A/A/B? What happens if a large number of candidates have difficulty with certain questions - do they moderate and adjust the boundaries accordingly?
DD is trying to work out how little work she can get away with in order to achieve an A, grrrrr

It's all based on UMS. 90 UMS is an A*, 80 an A etc. However this is a scaled mark and is adjusted accordingly to how hard the paper is. IIRC in DS's mock biology you needed 26/45 in core biology for an A and 31/45 in additional biology for an A. However this changes every year depending on how people score. I don't think I explained that very well!

yes, you have explained it exactly right! The UMS mark is fixed for a grade boundary. But the raw paper score that equates to the UMS mark does change year on year depending on how easy/difficult the paper was.

If this was AQA core then am I right in thinking the Biology felt more straight forward than the chemistry? (generally described as odd!)

Just interested to see if that was a feeling across the country and not just our school!

Thank you both for the explanation. It was AQA but she found a lot of unfamiliar questions in the Biology and thought the Chemistry was straightforward - though a lot of her friends found it tricky. Her revision seemed to consist entirely of old papers and looking up the answers she didn't know in a CGP revision book, for a couple of days before. Physics on Monday and she has done nothing so far. She is predicted A/A* but seems to do no work at home at all. She is doing the other papers in the summer so I hope she gets enough of a shock to spur her into action.