British Orienteering Federation Ranking List

Drifting Points

Since the inception of the Ranking List in 2000 the points awarded have been gradually drifting downwards. The original intention was to hold class averages at around 1000 points with winning scores typically up to about 1300 points. Drift has eroded the scores so that class means have fallen to around 800 points and winning scores barely more than 1100. Although some form of drift was anticipated at the outset, no attempt to renormalise the calculation was specified since it was felt better to see what was needed before trying to correct. We now understand the problem better and have removed the cause of the drift from the calculation. All event after 1 January 2002 now use the re-normalised calculation.

Cause of the Drift

There are two known causes for drift:

The current calculation has weighting factors imposed on people who take a long time (more than 175%) compared with the winner. When the weighting factor comes into play points are lost from the class causing a downwards drift. This is a systematic drift affecting all classes.
When someone joins or leaves a class the class average can change. The effect on the class average will depend on whether a person who leaves the class improved or got worse while a member of the class. The overall effect will be fairly random but the scores in younger age classes will tend to drift down while those of older age classes will tend to drift upwards. This random drift tends to be smaller than the systematic downwards drift which therefore dominates in most classes.

Fixing the Problem

Two fixes were needed.

The calculation had to be adjusted so that drift is eliminated in future calculations by incorporating a re-normalisation procedure in the score calculation for each event. This counteracts the effect of the loss of points caused by the weighting factor.
This years' scores have been re-normalised to bring the class means back to 1000. The adjustment needed to be event dependent since the total drift is worse now than it was a year ago so recent events will have a bigger boost to scores than older events. Inevitably this has produced some shuffling of places in the ranking list. Scores have risen by about 200 points on average but lower scores have received a bigger boost, while the rise for top scorers is less. Last years' scores have not been re-normalised.

A Temporary Glitch

For a few days during the change over period the weighting factor was omitted from the calculation. When it was observed that this omission produced a few anomolous results the weighting factor was re-instated.

What the IOF do

The IOF ranking system uses a similar calculation to ours but without the weighting factors (not applicable at world class level) so they are less prone to drift. An annual re-normalisation is carried out to remove any residual drift.

This page was last updated by Mike Cumpstey November 2002.