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Anyone good with Percentiles?

(30 Posts)
butterflymum Tue 05-Feb-13 10:31:10

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 Wed 06-Feb-13 13:02:08

(ps..... I would be first to admit I could very well be missing something obvious, as my head is buzzing with other issues at moment - I am only continuing with this query re the 61 as it has bugged me since I became aware of it and my curiosity is getting the better of me grin, so thanks again for all who have given input thus far thanks....and yes, I know, maybe I shouldn't be so curious and just go and have some biscuit's and get on with issues that really need my attention).

JoanByers Wed 06-Feb-13 13:18:20

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) <= sd <= 30

and therefore

89.13 <= %ile <= 95.94

lougle Wed 06-Feb-13 13:24:18

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

butterflymum Wed 06-Feb-13 14:14:03

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.

lougle Wed 06-Feb-13 14:30:19

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.

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