Gambling behaviours, defined in terms of frequency and amount of money lost, are important variables in gambling studies. The success of a certain form of therapy often depends on the reduction of expenditures related to gambling activities. Because gambling problems are largely related to monetary losses, this variable occupies a central place in prevalence studies and constitutes a social cost marker related to excessive gambling (Walker & Dickerson, 1996; Williams & Wood, 2004).

However, the evaluation of monetary gambling expenditures has not been standardized. Considerable differences exist between the participants' reported results (Blaszczynski, Dumlao, & Lange, 1997; Blaszczynski, Ladouceur, Goulet, & Savard, 2006). The fact that the majority of studies collect the data retrospectively could lead to biased measurements (Walker et al., 2004). The gamblers appear to estimate the amount spent during a single gambling session and then generalize it for all their sessions (Blaszczynski et al., 1997). This estimate is done subjectively, without necessarily using any kind of mathematical strategy that could be constant from one individual to another and from one study to another (Walker et al., 2004). Some authors suggest that using a daily self-reporting method would provide more valid data than would a retrospective report about expenditures related to gambling activities (Williams & Wood, 2004).

Another problem related to reporting gamblers' monetary expenditures is that few studies specify how to calculate the gambling expenditures. The ultimate goal is to know how much money was spent on gambling, that is, the difference between the amount of money at the beginning of the gambling session and the amount left at the end of the gambling session. In this regard, Blaszczynski et al. (1997) showed that the gambling expenditures reported by gamblers mostly depend on the decision whether to include or exclude the gains made during a gambling session.

The differences in the way that gamblers calculate the amount of money spent on gambling raises a problem. Blaszczynski et al. (2006) found that gamblers who calculate their gambling expenditures by including both the wins and the losses made during their gambling session usually report significantly higher expenditures than those who report the same expenditures using the “net expenditure strategy” (explained below).

The main objective of this study was to verify if the amount of money spent on gambling and calculated retrospectively for a monthly window differs from the (summed) amount recorded on a daily basis. The hypotheses were as follows:

1. 1. The monthly estimates of gambling expenditures will reveal different monetary amounts than those reported using the daily self-reporting chart.
2. 2. The amount revealed by calculating the total gambling expenditures will be different than gambling expenditures calculated using the net expenditure strategy.

The goal of this study was to evaluate the potential differences in gambling expenditures reported on a monthly basis, as compared with using a daily self-reporting chart. The hypothesis, which stated that global monthly evaluation of gambling expenditures will reveal different monetary amounts than those reported using the daily self-reporting chart, was confirmed. The results indicate that the monthly estimate of gambling expenditures was lower than that calculated using the sum of daily self-reporting expenditures. Gamblers under-estimated their gambling expenditures when they evaluated them on a monthly, as compared with a daily, basis. This result is the same as those previously reported by Walker et al. (2004), which suggested that the larger the temporal window, the more likely gamblers will give approximations that may not be valid. Although there is reason to question the validity of the data reported in studies about gamblers' expenditures, it is important to emphasize that no gold standard method has yet been established to collect the information concerning how much money a gambler spends in a given period.

The second hypothesis stated that the total gambling expenditures would be different if the net expenditure strategy was used. This hypothesis was confirmed. As expected, the total expenditure amount over the last month was higher than the amount calculated by taking the difference in the amount of money the gambler had on him/her before and after the gambling session. This finding runs parallel to that of Blaszczynski et al. (2006), where it was revealed that half of the gamblers included the profits that they made during a gambling session when indicating the amount of money they spent gambling. Thus, the amounts reported by the gamblers on a daily basis proved to be over-estimated in comparison to what the researchers really want to know, that is, gambling expenditures calculated using the net expenditure strategy. This high estimate could be the result of participants using the turnover strategy to report their expenditures (i.e., a strategy that incorporates the money “churned” while gambling into expenditure calculations) (Blaszczynski et al., 2006).

This study has one important limitation: About half of the participants dropped out. This number could be explained by the burden of the task, which consists of filling in the chart on a daily basis for a period of 28 days. However, the number of participants that quit the study is representative of the population being studied — active gamblers. Yet, 50 participants completed the self-reporting four charts for the entire duration of the study, without missing any information, and this is certainly not insignificant.

In conclusion, when gamblers are asked to report their gambling expenditures retrospectively for the preceding 1-month timeframe, they tend to under-estimate their expenditures. However, when asked to monitor and report their gambling expenditure on a daily basis, they have a tendency to over-estimate expenditures. These results as a whole indicate that there are inconsistencies in the literature that cast doubt on the validity of these data. We must therefore continue to be critical of data collected in response to items to the question: “How much money did you spend gambling?” When expenditure calculated on a daily basis was compared to an estimation for a 1-month period, the data did not concur. These data must be considered as indicators rather than as the gamblers' actual expenditures on games of chance and betting games. It is evident and urgent that this variable be examined in future research, and a gold standard delineated soon.