As we have mentioned before, one of our favourite sites for collecting historical data is www.oddsportal.com, and in our recent article about the concepts for football betting systems we outlined how to set-up an Oddsportal user account and how to optimise the settings.
In this article we concentrate on the rare occasions when it will be necessary to rectify errors in Oddsportal’s information to safeguard against distorted results. At the end, we offer a free Excel spreadsheet download to assist you with this process, plus an exercise to test what you have learned.
Oddsportal Archived Odds Overview
When clicking on any match in the Oddsportal “Archived Odds” section, you will be met with an image similar to the (cropped) screenshot on the right.
Click on the picture to enlarge it in a new tab and then use the magnifier provided to increase the image size further.
From first glance this appears to be a fairly ordinary summary of a match that finished 2-2, with a snapshot of the full-time home, draw, and away odds provided by a selection of traditional bookmakers.
At the bottom of each column is the average odds price from all the bookmakers and shown below this, the highest odds on offer.
In the extreme right-hand column is each bookmaker’s “payout” (not their overround) together with the average and highest figures.
All of the figures shown relate to the point in time when the 1×2 odds were suspended by each bookmaker as the match kicked off and went “in-play”.
But there is obviously something very wrong with this image.
Some of you will see the problem immediately but before we tell the rest of you what it is, try and work it out for yourself before moving onto the next page of this article…
Identifying False Football Betting Odds
The odds for the home win are distorted by a rogue price.
It will not take you long to see that the bookmaker, Sportingbet, is shown as offering odds at a price of 4.30, which is far in excess of the others. This is of course also distorting the average odds price, which is currently shown as 1.78.
The big giveaway is the “Payout” column on the right-hand side, which shows 149% for betting on the three highest prices for home, draw and away. This represents a huge arbitrage situation that is very unlikely to occur in 1×2 betting at a fixed point in time before kick-off (ante post betting).
The problem of false odds in an individual game may not sound serious but we have encountered a fair few of these situations during our data collection exercises with various sites and sometimes several in the same league in the same season. In the extreme examples, there is more than one bookmaker mistake in a single match.
Basing your 1×2 football betting system assumptions on a season’s worth of data with several rogue entries, some far worse than the one we have highlighted, may lead you into adopting a set of false belief systems about that particular league and its statistics.
Identifying potential problems therefore becomes necessary before working in any degree of confidence with the raw data.
If you remember from our football betting systems concepts article, we discussed the accuracy issue with mathematics in general.
Looking at a five-year analysis of a particular league’s statistics will sometimes not provide a huge pool of data (e.g. just 882 games in the Swiss Super League in the five seasons 2008-13) and it is here that accuracy with the information becomes more important.