Delay Discounting Task


Delay Discounting Task

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Jack
Jack
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Hi,


I'm using the Delay and Probability Discounting Task (Richards et. al., 1999) and was having trouble with some of the data. I've modified the script so that the reward values are all multiplied by a factor of five when the script is run. I've also removed the probability section of the task as well. However, the data will still show up as values between 1-10


I was having some trouble with the indifference point determination of the program.  For some subjects, even at time zero (no delay) people's indifference points show up as $9 rather than $10. This doesn't make sense at all because people don't discount when there's no delay, yet this result makes it appear that the individual is discounting $1 (or $5, according to what the subject is seeing). However, some subjects do show $10 at that IP with most don't. This poses a problem because I can't tell if somebody is actually discounting the $1 or if the program just says they are when they really aren't.


As far as I understand, the program determines the indifference point to be the immediate reward value of the question when the  bottom limit's maximum and top limit's maximum differ by 1 (the standard program differed by $0.5, but I changed this because we are not using $0.5 increments).


Is there any way to fix this? It just appears that the IP is not accurate and is randomly selected between the value of the bottom limit's maximum and the top limit's maximum.


I was also wondering how to determine the k value from the data. I'm currently using SPSS and I'm not sure if I can do it or how to do it through SPSS.  If there's any hints that would help a lot too!


Thanks,
Jack


Dave
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As far as I understand, the program determines the indifference point to be the immediate reward value of the question when the  bottom limit's maximum and top limit's maximum differ by 1 (the standard program differed by $0.5, but I changed this because we are not using $0.5 increments)


Yes, that is correct. Quoting from the script's comments:


   "As soon as the difference between the respective maximum bottom limit and maximum top
    limit reaches $0.50, the corresponding variable amount is recorded as estimate of the
    respective indifference point."


Is there any way to fix this? It just appears that the IP is not accurate and is randomly selected between the value of the bottom limit's maximum and the top limit's maximum.


That's how the Richards et al. adjustment procedure works [1] and as such there is nothing to fix. It's a constraint on the procedures resolution / measurement error. Of course, you could simply go ahead and always set the 0-delay IP to 10.


I was also wondering how to determine the k value from the data


You need a program that can do proper curve fitting (as detailed in Richards et al.). I don't think SPSS has that capability.



[1] Edited to add: It is, of course, conceivable that Richards et al.'s description of the adjustment algorithm is inaccurate and/or incomplete (i.e. contains ambiguities, bonafide mistakes or omits some important aspects). In other words: Perhaps the "true" algorithm ought to work differently in some fashion than the info available in their paper suggests. It is no less conceivable that my Inquisit implementation of said algorithm contains (a) mistake(s), despite my best efforts to the contrary. I am not aware of any, but that does not mean none exist. If you (or anyone else) should know of any such issue, I'd very much appreciate you pointing them out. Thanks!


flips333
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http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3405931/

To determine K 

GO


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