## Saturday, August 15, 2015

### Betting market update

Extracting the odds from bookmaker webpages is a dark art. The bookies make it hard to extract this data. For each bookie I need a bespoke web-scraper. While these only take a dozen lines of python code; each scraper is different. Furthermore, they are fragile and need to be reconfigured often as the bookmakers modify their web delivery strategies. Anyway, I have now written six little scrapers, which I run every morning.

This morning's odds follow. Links to these bookmakers can be found in the right-hand column.

House Coalition Odds ($) Labor Odds ($) Coalition Win Probability (%)
2015-08-15 CrownBet 1.65 2.25 57.692308
2015-08-15 Luxbet 1.66 2.20 56.994819
2015-08-15 TABtouch 1.76 2.04 53.684211
2015-08-15 Sportsbet 1.65 2.25 57.692308
2015-08-15 William Hill 1.77 2.05 53.664921

To calculate the Coalition's win probabilities from these odds, I use the following formula (where $$C$$ is the Coalition's odds and $$L$$ is Labor's odds). Of course, this formula simply normalises the probabilities for the bookmaker's over-round.

$p=\frac{\frac{1}{C}}{\frac{1}{C}+\frac{1}{L}}$

Looking back over the past week, all of the movement has been in Labor's favour (noting that I only ran scrapers for four bookies on 9, 10 and 11 August). On Monday, Sportsbet had reduced its Coalition win probability from 60.2 to 57.7 per cent. On Tuesday, TABtouch reduced from 57.7 to 53.7 and WIlliam Hill also reduced from 57.7 to 53.7 per cent. This morning, Luxbet was reduced from 57.7 to 57.0 per cent.

In charts, we can see the probability of a Coalition win at the next election has fallen from somewhere between 57 and 60 per cent to somewhere between 54 and 57 per cent.