it should be telling me how the school improves my child's performance
We get slightly more nuanced data these days, in addition to the school-wide VA. If you go here you can see how well the school does for low, middle and high attainers so you can see which statistic is relevant for your DC. You can also go to the Ofsted website and look up the Data Dashboard which shows how well the school is doing compared to others with similar intake.
Can anyone explain to me how the "value added" stat is calculated in the Ofsted reports. I understand that it should be telling me how the school improves my childs performance but what does it really mean and is it a reliable stat that should guide school choice??