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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 ).

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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