Report
Illegal online gambling: Challenges of estimating the size of the illegal gambling market
The final chapter of the Gambling Commission's research into the illegal online market, focusing on challenges estimating the size of the market.
Comparisons of different methodologies
Key point summary
Key points in this section are as follows:
- we have explored three main approaches to estimating the scale of the illegal online gambling market
- of these approaches, we have not pursued developing a survey-based methodology as we consider the underlying data to be too unreliableas we know that consumers’ recall of past expenditure in gambling surveys is generally poor: efforts to refine estimates through further assumptions are unlikely to lead to reliable results as the underlying core data is already flawed
- we are focusing on methodologies where assumptions can be clearly stated and improved over time as more data becomes available: this provides the opportunity of a pathway to more reliable estimates suitable for publication.
We have explored 3 main approaches to estimating the scale of the illegal online gambling market. In this section, we provide an overview of each and provide an overview of their strengths and weaknesses.
Dwell time approach – This converts data on engagement and ‘time-spent-on-site’ into expenditure estimates. This assumes that more visits to illegal websites, and longer visit duration, will be associated with higher levels of expenditure. Several assumptions must be made to generate financial estimates – this requires us to use data from known behaviour on legal websites and adjust this to develop an estimate of illegal gambling activity.
Channelisation approach – This is based on making a comparison between data on legal and illegal ‘channels’ of engagement with gambling. Our Illegal online gambling: Consumer engagement and trends report sets out our approach to estimating consumer engagement with illegal websites. This approach can also be used to obtain estimates of engagement with legal websites. The 2 datasets can then be benchmarked with each other. As we have data on levels of Gross Gambling Yield (GGY) associated with the legal market, this can be used as a starting point for making an estimate of GGY associated with illegal activity.
Survey based approach – Consumer surveys can be used to ask consumers directly about their past experiences of using illegal sites. Results can be scaled up to generate a population level estimate of consumer expenditure on illegal websites.
Strengths and weaknesses
Dwell-time approach
Strengths
This approach is based on objective measures of estimated engagement with illegal gambling websites. Our approach to generating these estimates is set out in Report 2 Our Illegal online gambling: Consumer engagement and trends report. Although initial estimates of engagement are already subject to margins for error, these margins have been estimated and quantified using statistical techniques. This allows uncertainty to be better understood and communicated.
Assumptions can then be applied to provide a more refined estimate of expenditure on these sites. These assumptions can be improved as more data becomes available – allowing us to move towards a more reliable estimate.
Weaknesses
This approach requires multiple assumptions to convert estimates of engagement into estimates of expenditure – such as differences in levels of expenditure between legal and illegal gambling and the proportion of visit time to a website which is actively spent gambling.
Each additional assumption adds additional margins for error – collectively these can add up to create significant uncertainty over the estimates.
Channelisation
Strengths
As with the ‘dwell time’ approach, the foundation for this is objective data on estimated engagement with legal and illegal gambling.
Trends in illegal engagement can be compared with data with more established understanding of consumer activity in the legal gambling market.
Assumptions must still be applied to generate reliable estimates. These can be set out and improved as more data becomes available.
This approach also allows us to observe and compare seasonality in data in comparison to more established trends in the legal market.
Weaknesses
This approach also requires multiple assumptions which, when combined, introduce significant margins for error.
Survey-based methodology
Strengths
Survey data can be easily collected – making it a relatively low-cost approach to developing an estimate.
Weaknesses
We know that consumers recall of gambling expenditure is unreliable1. For this reason, we do not ask respondents about past expenditure in the Gambling Survey for Great Britain (GSGB). Nor were questions used in the previous Health Surveys.
Assumptions still need to be applied to scale up survey responses – but as these assumptions must be applied to already unreliable data, we consider the overall uncertainty of any estimates generated through this approach to be extremely high.
In addition to concerns about reliability of recall, obtaining a representative sample of consumers who have used the illegal market is challenging – especially as we know from past research that many consumers do not know if the websites they are using are regulated in Great Britain or not.
Conclusions on methodologies to develop further
In summary, we have focused on exploring ‘dwell time’ and ‘channelisation’ approaches to understanding the size of the illegal online gambling market as likely to provide a more robust estimate.
References
1 Accuracy of self-reported gambling frequency and outcomes: Comparisons with account data (opens in new tab); Heirene, Wang, Gainsbury; Psychology of Addictive Behaviours; 2021.
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Dwell-time approach - Challenges of estimating the size of the illegal gambling market
Last updated: 6 November 2025
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