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 Post subject: 2014 CEM - A Statistical PerspectivePosted: Thu Oct 16, 2014 1:17 pm

Joined: Fri Mar 04, 2011 12:41 pm
Posts: 158
Disclaimer: The author bears no responsibility for any decisions made based on any information or opinion contained herein

This post summarises my attempts at trying to analyse the limited information on CEM results. The Admissions Office at CCHS has not been helpful with my request for some basic information, unlike the CSSE office. Hence I have no actual facts to use, but can only extrapolate based on the limited information in the CCHS letter and some of the scores and cut offs for 2014 entry and 2015 entry posted in this forum.

Some key points, best taken with a large dose of salt:
1. I believe the CEM test scores are standardised with a mean score of 100

2. The distribution of scores is broadly normal, but is skewed towards scores being slightly below average offset by a small number of very high scores

3. The range of scores for 2014 entry was 50 to 143 and for 2015 entry 66 to 138. This is not to say anyone necessarily scored 66 or 138, but those were the minimum and maximum scores theoretically possible

4. Working backwards and trying to match the proportion of scores in the top bracket, statistically speaking the standard deviation for 2014 entry was 10.7 and for 2015 entry 12.6

5. Assuming the cutoff score for in catchment place in 2014 was around 107, this would have been around rank 200 out of 848

6. Extrapolating 2014 into 2015 means that a in catchment score of 108.5 would result in a rank around 200 and should be sufficient for an offer. Scores slightly below 108.5 could still be successful but this starts getting close to the borderline

7. If the cut off score for out of catchment places in 2014 entry was around 116, this would be rank 57 out of 848

8. Applying the same ratio for 2015 entry means a successful out of catchment score would be around 118.50

9. Because there is a maximum of only 24 places for out of catchment places, the statistical model is less useful in predicting the cut offs and individual preferences by the high scores become very significant

10. These points are based on a statistical model which has significant limitations and limited predictive power. Caveat emptor and Good Luck!

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 Post subject: Re: 2014 CEM - A Statistical PerspectivePosted: Thu Oct 16, 2014 3:11 pm

Joined: Sat Oct 11, 2014 4:56 pm
Posts: 18
Wow. Aang, thanks yet again for a detailed response. DD got 112 and OOC so not looking good from your explanation. But who knows........

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 Post subject: Re: 2014 CEM - A Statistical PerspectivePosted: Thu Oct 16, 2014 9:50 pm

Joined: Fri Mar 07, 2014 6:50 am
Posts: 69
aang wrote:
Disclaimer: The author bears no responsibility for any decisions made based on any information or opinion contained herein

This post summarises my attempts at trying to analyse the limited information on CEM results. The Admissions Office at CCHS has not been helpful with my request for some basic information, unlike the CSSE office. Hence I have no actual facts to use, but can only extrapolate based on the limited information in the CCHS letter and some of the scores and cut offs for 2014 entry and 2015 entry posted in this forum.

Some key points, best taken with a large dose of salt:
1. I believe the CEM test scores are standardised with a mean score of 100

2. The distribution of scores is broadly normal, but is skewed towards scores being slightly below average offset by a small number of very high scores

3. The range of scores for 2014 entry was 50 to 143 and for 2015 entry 66 to 138. This is not to say anyone necessarily scored 66 or 138, but those were the minimum and maximum scores theoretically possible

4. Working backwards and trying to match the proportion of scores in the top bracket, statistically speaking the standard deviation for 2014 entry was 10.7 and for 2015 entry 12.6

5. Assuming the cutoff score for in catchment place in 2014 was around 107, this would have been around rank 200 out of 848

6. Extrapolating 2014 into 2015 means that a in catchment score of 108.5 would result in a rank around 200 and should be sufficient for an offer. Scores slightly below 108.5 could still be successful but this starts getting close to the borderline

7. If the cut off score for out of catchment places in 2014 entry was around 116, this would be rank 57 out of 848

8. Applying the same ratio for 2015 entry means a successful out of catchment score would be around 118.50

9. Because there is a maximum of only 24 places for out of catchment places, the statistical model is less useful in predicting the cut offs and individual preferences by the high scores become very significant

10. These points are based on a statistical model which has significant limitations and limited predictive power. Caveat emptor and Good Luck!

Hi, firstly I would like to say Welldone for the immense amount of time and feedback you give here. like all of us I've been questioning, trying to get answers that would help me move on with the CAF application!

Aang
I would be super grateful if you could advise me if a should not put Chelmsford (CCHS ) as an option. 95% of me says not to as my dd's score is in the second band and there are so many fantastic high scores above her. Another school can be entered instead of CCHS on the application.
Her score is 96.09 (91.01-104) we are OOC .

Now what confuses me is what is stated on the results letter that:

Do not worry if your daughter is not in the top range as not everyone who sat the test formally applies for a place

Are they referring to the people inside catchment, or both? I have already counted 10 high OOC results that want CCHS as their first choice, so shall a leave this completely off my list or do as they say to apply being in the second band? I would feel crazy to do this as I feel we won't have a chance, but statically I'm really not sure?

Why would they say that? The two bands make up 600 girls. Would love to hear what you think? Thankyou

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 Post subject: Re: 2014 CEM - A Statistical PerspectivePosted: Fri Oct 17, 2014 1:00 am

Joined: Fri Mar 04, 2011 12:41 pm
Posts: 158
A score of 96 will not get an offer from CCHS even if you were in catchment. You do have plenty of schools to put on your CAF form, so one nomination of CCHS at #1 still allows you to choose plenty of other schools and you can sleep easy knowing you tried. You would also find out in March where you are ranked in the waiting list. My estimate is that 96 would rank around 470 out of 800 who took the CEM.

For some reason, CCHS is encouraging people to include CCHS even if they are in the second band. There is probably ulterior motives behind this to give support that CCHS remains a popular and attractive school, but from the statistics that I have seen, I think you really have to be in the top band to have a chance of an offer. If you are out of catchment, then you probably need a high top band score per my summary above.

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 Post subject: Re: 2014 CEM - A Statistical PerspectivePosted: Fri Oct 17, 2014 6:20 am

Joined: Fri Mar 07, 2014 6:50 am
Posts: 69
Thankyou Aang , you've helped me ease my decision, because for the Redbridge intake my DD was advised not to consider a place as she was outside of the 171 ranking. They made it clear. She ranked 361 score was 100.11 for woodford County. As it was the same exam I knew she would not had made both.

comparing the CCHS/96.09 and woodford/ 100.11 they have a difference in 4 marks.
I assumed that the first 300 girls in the top range equals to the 300 in her score and the 61 +4 would come off the second band, hence making her closer to the top band. I hope this makes sense.

I think CCHS really could have made their ranking more clearer like Redbridge have. Esp with the range looking at past years. Also agree to make themselves more popular in numbers of applications.

Thanks again , appreciate you help

Last edited by Icepop on Fri Oct 17, 2014 8:46 am, edited 1 time in total.

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 Post subject: Re: 2014 CEM - A Statistical PerspectivePosted: Fri Oct 17, 2014 8:30 am

Joined: Sat Oct 11, 2014 11:41 pm
Posts: 3
Aang is it possible to estimate the impact of age standardisation on the scores? For example what would be the difference for the same mark for a September born versus a march born?

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 Post subject: Re: 2014 CEM - A Statistical PerspectivePosted: Fri Oct 17, 2014 11:01 am

Joined: Fri Mar 04, 2011 12:41 pm
Posts: 158
Chicken soup

Sorry, it is not possible for me to estimate the impact of age standardisation on the CCHS CEM scores. You would need many candidate's raw scores and birthdays to be able to work backwards.

Okanagan wrote a good post on standardisation with a paragraph on age standardisation in this thread
viewtopic.php?f=37&t=27415&start=94

There is also a sticky with a more detailed explanation on standardisation at http://www.elevenplusexams.co.uk/advice ... xplanation

Most other CEM schools give an age standardised score without any decimal figures. I do not know how CCHS derives their figures and CCHS have not been helpful in answering my requests for information. Anecdotally from other CEM results in other parts of the country, age standardisation may affect a score by a few percent (typically less than 5%).

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 Post subject: Re: 2014 CEM - A Statistical PerspectivePosted: Fri Oct 17, 2014 1:53 pm

Joined: Sat Oct 11, 2014 11:41 pm
Posts: 3
Thank you for that info aang, v interesting as always! It was purely out of interest that I was asking, would not have made any difference in our case to getting in or not.

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 Post subject: Re: 2014 CEM - A Statistical PerspectivePosted: Fri Oct 17, 2014 8:14 pm

Joined: Fri Oct 17, 2014 6:25 am
Posts: 5
Hi Aang,
Thank you for your statistical inferences. Please can you indicate where you think the rank is for a score of 111 & 112. Many thanks.

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 Post subject: Re: 2014 CEM - A Statistical PerspectivePosted: Fri Oct 17, 2014 11:36 pm

Joined: Fri Mar 04, 2011 12:41 pm
Posts: 158
Based on a mean of 100 and a standard deviation of 12.6, a score of 112 would be ranked around 137 out of 100, and 111 would rank 153 out of 100. These are ballpark numbers at best as there are limitations of the statistical model, but I would expect these scores to result in an offer at CCHS as long as they are in catchment candidates..

Interested2014 wrote:
Hi Aang,
Thank you for your statistical inferences. Please can you indicate where you think the rank is for a score of 111 & 112. Many thanks.

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