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Mark Griffiths



You can read the original study as a pdf here.



And here is Professor Mark Griffiths' excellent blog on all things related to addiction.



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The Griffiths Page

Griffiths, M. D. (1994) The role of cognitive bias and skill in fruit machine gambling. British Journal of Psychology. 85. 351-369

On this page you should find lots of useful stuff to help you in your learning of Griffith's gambling study.

Here is the most important page. Click here for a summary and evaluation of the gambling experiment.

This page has some Core Studies Section A past questions that you might want to practice.

Here is a multi-choice quiz to test your knowledge of the study. Thanks Kim.

And here is a great page on Jamie?s psychblog where you can read and listen to more about this study.



Below is a much briefer summary of the Griffiths experiment.



In this study Mark Griffiths is investigating some of the cognitive differences between regular and non-regular gamblers. In particular he is interested in discovering whether regular fruit machine payers think differently to non-regular players. That is, whether regular fruit machine players display cognitive distortions.

Wagenaar argues that gamblers use a range of cognitive distortions called heuristics.



The aim of this study was to investigate cognitive bias involved in gambling behaviour.

There were three main hypotheses

There would be no difference between regular and non regular fruit machine gamblers on objective measures of skill.

Regular gamblers would produce more irrational verbalisations than non-regular gamblers

Regular gamblers would be more skill orientated than non-regular gamblers on subjective measures of self-report

A further hypothesis was also added that thinking aloud participants would take longer to complete the task than non-thinking aloud participants.



This study is using a quasi experimental approach. The main independent variable is whether participants are regular or non-regular gamblers.

The dependent variables are objective measurements of skill on the fruit machine, the content analysis of utterances from the thinking aloud method and the subjective measures of skill perception from the post-experimental semi-structured interview.

A further independent variable was created by a half of both the regular and non-regular gamblers being randomly assigned to the ?thinking aloud? condition.

The sample consisted of 60 participants. Most of the participants were recruited via a small poster advertisement circulated around a university and college campus. A number of the regular players were recruited via a regular gambler known to Griffiths.

The regular gamblers consisted of 29 males and 1 female and the mean age was 21.6 years.

The non regular gamblers consisted of 15 males and 15 females and the mean age was 25.5 years.

Each participant was tested individually at an arcade in the UK. All of the participants were asked to gamble on a fruit machine called ?Fruitskill? which played at 10 pence a go.

Each participant was given ?3 to gamble on the fruit machine which gave them 30 ?free? gambles.

All of the participants were asked to try and stay on the machine for a minimum of 60 plays ? which means that had to break even and win back ?3 from the money they had put in.

If they managed to achieve 60 gambles with the initial ?3 stake they were given the choice of either keeping any of the winnings or carry on gambling.

To measure the participants objective skill levels, Griffiths recorded the gamblers behaviour.

The experimenter stood nearby recording the total time in minutes each participant was on the fruit machine, the total number of gambles, the amount of winnings and the result of every gamble.

To measure the participants irrational verbalisations, Griffiths devised the ?thinking aloud? technique.

The regular and non-regular fruit machine gamblers were randomly assigned to either the ?thinking aloud? condition or ?non-thinking aloud? condition.

The participants assigned to the thinking aloud condition had their verbalisations recorded using a lapel microphone connected to a portable tape recorder. The experimenter transcribed all of the verbalisations made during the gambling session within 24 hours so that he could remember the context of the verbalisations.

A content analysis was performed on the transcriptions. Based on the utterances Griffiths created a coding scheme with 30 irrational and irrational utterances and a further miscellaneous category.

A post experimental semi-structured interview was also carried out to measure the participants? subjective opinions of their skill levels.

A number of questions relating to skill were asked. For example participants were asked ?Is there any skill involved in playing a fruit machine? And ?How skilful do you think you are compared with the average player?? What skill (if any) is involved in playing fruit machines?



Griffiths reported the results in three parts relating to each of the three main hypotheses.

Analysis of the behavioural data.

It was found that on the whole were was no difference in objective measures of skill between regular and non regular fruit machine gamblers.

This supports the first hypothesis that there would be no difference between regular and non regular fruit machine gamblers on objective measures of skill.

In relation to the fourth hypothesis the thinking aloud participants did take longer to complete the task than non-thinking aloud participants, although this was not significantly longer.

Analysis of the verbalisations

Using the ?thinking aloud method? it was found that regular gamblers did make more irrational verbalisations than non regular gamblers. 14 percent of the regular gamblers verbalisations were irrational comments compared to 2.5 percent of the non regular gamblers.

The results also showed that regular gamblers made significantly more percentage verbalisations in just two of the categories ? these were personifying the machine (e.g. ?the machine likes me?) and referring to the ?number system? (e.g. ?I got a 2 there?).

Non regular gamblers made significantly more verbalisations in questions relating to confusion and non-understanding, in statements relating to confusion and non understanding and miscellaneous utterances.

Analysis of skill variables

Based on the findings of the post-experimental semi-structured interview it was found that regular gamblers were more skill orientated than non-regular gamblers.

In response to the question ?How skilful do you think you are compared with the average person?? regular gamblers claimed that they were at least of average skill, but more usually ?above average skill? or ?totally skilled?. Non regular gamblers an the whole viewed themselves as ?below average skill? or ?totally unskilled?



Griffiths argues that this study shows that regular fruit machine gamblers are not significantly more skilled on fruit machines than non regular gamblers and that regular gamblers believe that their activity is far more skilled than it actually is.

The study supports the argument that regular fruit machine users do use cognitive biases when gambling. Griffiths argues that although regular gamblers do make more irrational verbalisations he is cautious about whether such findings do explain that the difference between regular and non-regular gamblers. Griffiths argues that more research needs to be carried out to discover whether the choice of heuristics is the underlying cause of irrational gambling behaviour or whether the choice of heuristics are the symptoms of a deeper underlying cause such as personality defects.

Griffiths argues that knowledge of the heuristics gamblers use could be used to rehabilitate gamblers through cognitive behavioural modification. This would involve modifying the thought patterns of an individual in an attempt to moderate or stop their gambling. Griffiths has termed this technique ?audio playback therapy? and provides anecdotal evidence of its use.