Caveats first: these conclusions, such as they are, are drawn from a small sample of a little over 50 schools. That sample of schools isn’t representative: indeed, it has slightly higher attainment than the national picture, both in terms of KS2 outcomes, and in KS1 starting points. However, with over 2000 pupils’ data, it shows some interesting initial patterns – particularly when comparing the three subject areas.
Firstly, on Maths – the least controversial of the three subjects. It seems that – in this sample – pupils who achieved Level 2c at KS1 had an approximately 40% chance of reaching the new expected standard (i.e. a scaled score of 100+). That leaps to around 66% for those achieving L2b at KS1 (i.e. just short of the national average)

The orange bar shows the average of this sample, which is slightly higher than the national average of 70%
It’s important to note, though, that progress measures will not be based on subject levels, but on the combined APS score at Key Stage 1. The graph for these comparisons follows a similar pattern, as you’d expect:

Where fewer than 10 pupils’ data was available for any given APS score, these have been omitted.
There is an interesting step here between pupils in this sample on APS of 13 (or less) who have a chance of 40% or less of reaching the expected standard, while those scoring 13.5 or more have a greater than 50% chance of achieving the standard. (The dip at 12.5 APS points relates to pupils who scored Level 2s in Maths and one English subject, but a level 1 in the other, highlighting the importance of good literacy for achievement in KS2 Maths)
For Reading, the graphs look broadly similar in shape

Blue bar shows average of this sample at 67%, which is slightly higher than national average of 66%
Interestingly here the level 2c children scorers still have only 40% chance of meeting the expected standard, but those achieving 2b have a lower chance than in maths of reaching the expected standard (58% compared to 66% for Maths).
When looking at the APS starting points, there is something of a plateau at the right-hand end of the graph. The numbers of pupils involved here are relatively few here (as few as 31 pupils in some columns). Interestingly, the dip at 18.5 APS points represents the smallest sample group shown, made up of pupils who scored 2a/2b in the two English subjects, but a Level 3 in Maths at Ks1. This will be of comfort to teachers who have been concerned about the negative effect of such patterns on progress measures: it seems likely that we will still be comparing like with like in this respect.
It is in Writing that the differences become more notable – perhaps an artefact of the unusual use of Teacher Assessment to measure progress. Compared to just 40% of pupils attaining L2c in Reading or Maths achieving the new expected standard, some 50% of those in Writing managed to make the conversion, and this against a backdrop of teachers concerned that the expected standard was too high in English. Similarly, over 3/4 of those achieving Level 2b managed to reach the standard (cf 58% Reading, 66% Maths)

In contrast to the other subjects, attainment in this sample appears notably lower in Writing than the national average (at 70% compared to 74% nationally)
With the APS comparisons, there are again slight dips at certain APS points, including 18.5 and 19.5 points. In the latter case, this reflects the groups of pupils who achieved Level 3s in both Reading and Maths, but only a 2b in Writing at KS1, suggesting again that the progress measure does a good job of separating out different abilities, even using combined APS scores.
Of course, this is all of interest (if you’re interested in such things), but the real progress measures will be based on the average score of each pupil with each KS1 APS score. I’d really like to collect some more data to try to get a more reliable estimate of those figures, so if you would be willing to contribute your school’s KS1 and KS2 data, please see my previous blog here.
Spread of data
Following a request in the comments, below, I’ve also attached a table showing the proportions of pupils achieving each scaled score for the two tests. This is now based on around 2800-2900 pupils, and again it’s important to note that this is not a representative sample.