[senco-forum] Urgent question re statistics please

Hi Mary

  No expert but this is what I think.
  Yes, a difference of 15 points is a standard deviation and is statistically significant.

  A small improvement in standardised score is a big improvement in percentile rank.  Examples:  SS 80  PR 9  SS 95  PR 37  SS 110 PR 74
  So a difference of 15 points in SS makes a completely different difference in PR.

  To me, it is all about what you want to prove and who to.  I'd use the one which proved that I'm making a big difference to the child if I want to prove that the programme I'm using is working well or the one which showed that, despite my best efforts, the child has made little or no improvement if I need to show that.
  Am a a pragmatist or just cynical?
  Amanda

Mary Kelly <mary.kelly4 at ntlworld.com> wrote:
  Could someone out there who is more statistics-literate than I am please
answer these two questions:
1. Am I right in thinking that a difference of 15 points in
standard scores is a standard deviation and is statistically
significant?
2. Am I right in thinking that it is NOT right to plot percentiles
as a graph because it would exaggerate any differences and make them
look more significant than they really are. That is, graphs should show
standard scores rather than percentiles?
I’d be really grateful for an answer tonight if at all possible please?
Many thanks,
Mary

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Amanda
Secondary SENCO
Cornwall