Anyone good with Percentiles?

[butterflymum]

I understand the basics of Standardised Age Scores and Percentiles, but this example has me confused (brain tired with other matters perhaps). Anyhow, thoughts welcome:

(working on basis of 69/70 to 140/141 with 100 as average)

SAS for English 115
SAS for Maths 122
Combined SAS 237 (maximum possible 282)

I would assume individual percentiles to be 84th percentile for the 115 and 93rd percentile for the 122 (using www.nfer.ac.uk/nfer/research/assessment/eleven-plus/standardised-scores.cfm ).

How is percentile worked out on the Combined SAS though?

butterflymum, if the English and Maths scores are identical for any given randomly selected individual, then yes the standard deviation would double. If the two scores vary for a given individual, the standard deviation would increase by less than double, in which case obviously a score of 237 is even beyond the 89.13th percentile.

We know:
15*sqrt(2)

Can the people who gave you the example not explain the fact that the combined is 61?

Their apparent answer was the one I posted a few posts back, lougle, and which I think was 'fudging' the issue a bit. They did not, it would seem, back it up with chart/tables to justify the 61.

See, if the combined percentile was much higher than the individuals, then I could understand that, with the logic of the probability of having a child who was both excellent at maths and english being lower than one or the other

eg. 1/3 good at english, 1/3 good at maths, 2/3 good at english or maths, but only 1/9 being good at english and maths.

But for the percentile to be much lower....doesn't add up.