Radio 4, More or Less program "Regression to the mean"
Discussion
Caught this on R4 More or less program. Thought it might be of interest to some. Or not.
Listen again section for latest (todays date should you be looking at this tomorrow) program. 11:13 in to the program.
Brought about by Monboits comments apparently??
Linky
http://news.bbc.co.uk/1/hi/programmes/more_or_less...
Listen again section for latest (todays date should you be looking at this tomorrow) program. 11:13 in to the program.
Brought about by Monboits comments apparently??
Linky
http://news.bbc.co.uk/1/hi/programmes/more_or_less...
Edited by jmorgan on Monday 19th November 18:36
k all,the regression to mean calculations where done on 30 limit roads, which IF the limit is being used correctly are roads with the highest density of potentitial collisions.I've heard about this regression to mean rubbish before.
I couln't find the article but I guess its the idea where road speeds are reclassified to their current average measured speed.
Ie if a current 60 limit road is measured and its found the average speed of traffic on that road is actually 40 then the limit for that road will be reduced to 40.
You could say thats sensible as we may possibly if that is applied to all roads get a raising of the limit on motorways and not before time..
However as allways with any govt policy I can spot the fatal flaws after considering the problem for a whole er three seconds.
Consider this.
The average speed along any route if measured represents just that.
some traffic may be travelling at the current speed limit..say 60 and others dawdling at 40. you would end up with an average of 50mph so you would reclassify that road as a 50 road and that would seem to be a job well done..
Except.... complete lack of intelligence has been used.
consider the new 50 road..
now we have traffic travelling at 50 (the new speed limit) and we still have people dawdling at 40mph therefore the new average speed of this road is now 45mph..
notice the difference we have ended up slowing the average speed of traffic down by 5mph.. as if roads weren't congested enough.
The financial impact on the economy for huge changes as these will run into billions.
As a simple example I used to run a factory delivering furniture. It made two deliveries to a client 75 miles away. you could get two round trips completed in one working day.
a reduction overall of 5mph would have meant my second delivery would have been around 30 miles away from my customer for the final delivery. and would have arrived too late to make the second drop.
For me to make sure I got two deliveries in each day I would then have to lease another vehicle and employ another driver while bothe drivers in effect did just over half a days work each.
The government dont take these sort of issues into account and should instead be making sure they do all they can to speed road averages up.
idiots
I couln't find the article but I guess its the idea where road speeds are reclassified to their current average measured speed.
Ie if a current 60 limit road is measured and its found the average speed of traffic on that road is actually 40 then the limit for that road will be reduced to 40.
You could say thats sensible as we may possibly if that is applied to all roads get a raising of the limit on motorways and not before time..
However as allways with any govt policy I can spot the fatal flaws after considering the problem for a whole er three seconds.
Consider this.
The average speed along any route if measured represents just that.
some traffic may be travelling at the current speed limit..say 60 and others dawdling at 40. you would end up with an average of 50mph so you would reclassify that road as a 50 road and that would seem to be a job well done..
Except.... complete lack of intelligence has been used.
consider the new 50 road..
now we have traffic travelling at 50 (the new speed limit) and we still have people dawdling at 40mph therefore the new average speed of this road is now 45mph..
notice the difference we have ended up slowing the average speed of traffic down by 5mph.. as if roads weren't congested enough.
The financial impact on the economy for huge changes as these will run into billions.
As a simple example I used to run a factory delivering furniture. It made two deliveries to a client 75 miles away. you could get two round trips completed in one working day.
a reduction overall of 5mph would have meant my second delivery would have been around 30 miles away from my customer for the final delivery. and would have arrived too late to make the second drop.
For me to make sure I got two deliveries in each day I would then have to lease another vehicle and employ another driver while bothe drivers in effect did just over half a days work each.
The government dont take these sort of issues into account and should instead be making sure they do all they can to speed road averages up.
idiots
mark69sheer said:
I've heard about this regression to mean rubbish before.
I couln't find the article but I guess its the idea where road speeds are reclassified to their current average measured speed.
No, regression to the mean is a statistical measurement tool. It is used to correct data collection from blips and anomolies.I couln't find the article but I guess its the idea where road speeds are reclassified to their current average measured speed.
If say, we were studying a road for 10 years and in 9 of the years there was no accidents but on one year 2 coaches collided, killing 50 people. The results would say an average of 5 deaths/year. This incorrect as really there is no history of fatal accidents on this road, just a very big one off accident.
Regression to the mean would take this blip into account and dismiss it.
mark69sheer said:
I've heard about this regression to mean rubbish before.
I couln't find the article but I guess its the idea where road speeds are reclassified to their current average measured speed.
Nope, wrong end of the stick entirely - it's a statistical tool which aims to factor out extremes in result sets.I couln't find the article but I guess its the idea where road speeds are reclassified to their current average measured speed.
Sorry I am totally wrongposted in here then.
I thought this was a reference to the proposed scheme to measure the average speed of roads and set their speed limits accordingly.
It was mentioned last year when a wholesale reappraisal of every road was suggested more or less doing away with the NSL outside restricted areas but instead reclassifying every road.
I suspect this scheme will never take off anyway as the cost of measuring and resigning would be too high.
I thought this was a reference to the proposed scheme to measure the average speed of roads and set their speed limits accordingly.
It was mentioned last year when a wholesale reappraisal of every road was suggested more or less doing away with the NSL outside restricted areas but instead reclassifying every road.
I suspect this scheme will never take off anyway as the cost of measuring and resigning would be too high.
mark69sheer said:
Sorry I am totally wrongposted in here then.
I thought this was a reference to the proposed scheme to measure the average speed of roads and set their speed limits accordingly.
It was mentioned last year when a wholesale reappraisal of every road was suggested more or less doing away with the NSL outside restricted areas but instead reclassifying every road.
I suspect this scheme will never take off anyway as the cost of measuring and resigning would be too high.
Its happening in Lincolnshire as we speak. Doing the main roads around Lincoln, A46, A15 and a few others.I thought this was a reference to the proposed scheme to measure the average speed of roads and set their speed limits accordingly.
It was mentioned last year when a wholesale reappraisal of every road was suggested more or less doing away with the NSL outside restricted areas but instead reclassifying every road.
I suspect this scheme will never take off anyway as the cost of measuring and resigning would be too high.
mark69sheer said:
Sorry I am totally wrongposted in here then.
I thought this was a reference to the proposed scheme to measure the average speed of roads and set their speed limits accordingly.
It was mentioned last year when a wholesale reappraisal of every road was suggested more or less doing away with the NSL outside restricted areas but instead reclassifying every road.
I suspect this scheme will never take off anyway as the cost of measuring and resigning would be too high.
I think you are probably referring to the 85%ile.I thought this was a reference to the proposed scheme to measure the average speed of roads and set their speed limits accordingly.
It was mentioned last year when a wholesale reappraisal of every road was suggested more or less doing away with the NSL outside restricted areas but instead reclassifying every road.
I suspect this scheme will never take off anyway as the cost of measuring and resigning would be too high.
This is a tool used by many involved in road engineering. A random sample of vehicles are measured along a road, and the speed up to which 85% of them are travelling is calculated. This, along with other factors (something often missed in discussions) is then used to help with setting the limit for the road.
RTTM is a statistical reference. Imagine a group of students taking a test. Scores vary from 5% up to 95%, with the average at 50% (most in the midlle). The scores are achieved by a combination of knowledge, luck, spelling errors, health, alcohol consumption, etc. So those with the lower scores could get a higher score another day. If you took a sample of students from the lower end, and retested them after giving them extra tuition, the average score of that sample will probably have moved up, nearer to the original average.
Just like the 85%ile isn't the full answer to setting speed limits, RTTM isn’t the full answer to explaining away any claimed gains.
There is a nice balance, regardless of what some will tell you! Oh, and RTTM doesn’t explain away the 2 crashes in the 10th year either.
BFF
Edited by Big Fat F'r on Tuesday 20th November 13:08
I don't think he is. I have personal experience of local roads being set to the average speed, not the 85%.
A6144(M) - formerly 70mph single carriageway, now 50mph. The email response said that the average speed was 51mph or so, and new rules allowed them to set the speed limit to within 10% of the average speed.
They completely ignored the fact that many people incorrectly presumed the road was 40mph, and also that many HGVs also used the road, and that the measuring strips were about 100 yards away from the traffic lights.
A6144(M) - formerly 70mph single carriageway, now 50mph. The email response said that the average speed was 51mph or so, and new rules allowed them to set the speed limit to within 10% of the average speed.
They completely ignored the fact that many people incorrectly presumed the road was 40mph, and also that many HGVs also used the road, and that the measuring strips were about 100 yards away from the traffic lights.
Parrot of Doom said:
I don't think he is. I have personal experience of local roads being set to the average speed, not the 85%.
A6144(M) - formerly 70mph single carriageway, now 50mph. The email response said that the average speed was 51mph or so, and new rules allowed them to set the speed limit to within 10% of the average speed.
They completely ignored the fact that many people incorrectly presumed the road was 40mph, and also that many HGVs also used the road, and that the measuring strips were about 100 yards away from the traffic lights.
'Tis indeed a strange and bizarre world we live in.A6144(M) - formerly 70mph single carriageway, now 50mph. The email response said that the average speed was 51mph or so, and new rules allowed them to set the speed limit to within 10% of the average speed.
They completely ignored the fact that many people incorrectly presumed the road was 40mph, and also that many HGVs also used the road, and that the measuring strips were about 100 yards away from the traffic lights.
At least they didn't try and explain it away with RTTM.
BFF
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