 |
 |
Preston Long writes:
> What are your thoughts on the outcome importance of two identical
> studies when one is single blinded and the other double blinded ?
> What action steps could be taken to reduce bias in the single
> blind study?
There is a bit of ambiguity in the terminology of single and double blind, so make sure you specify who knew what and when they knew it.
I have an example of a single blind study gone bad on my web pages:
http://www.childrens-mercy.org/stats/journal.asp (look at
Chapter 3). I'm currently updating this material, so any feedback would be appreciated.
It would be nice if you could make a quantitative adjustment after the fact (e.g., single blind studies overstate the effect size by 25%) but the latest research suggests that the effect of lack of blinding are variable and unpredictable.
During the study, there are some steps you should take. First, minimize the amount of interaction between the blinded patients and the medical personnel who are unblinded. Or, if the patients are unblinded and the medical personnel are blinded, then inform the patients that they should not discuss which treatment they received.
Second, try to get blinded evaluations of some or all of the end points that might be subjective. So, for example, have someone other than the surgeon assess the success or failure of the surgery.
Third, ask the individuals who were blinded if they know or suspect the treatment group assigned to a particular case. If only 50% respond correctly, then you know that no one has consciously or subconsciously passed on information.
Steve Simon, [log in to unmask], Standard Disclaimer.
The STATS web page has moved to
http://www.childrens-mercy.org/stats.
P.S. There's an interesting story about patients in one of the earlier AIDS trials who would take their first batch of medication to a chemist for analysis. If they found out that they were getting a placebo, they would drop out of the study and re-enroll under a different name. I've lost the citation for this anecdote, so if anyone can supply me with a reference, I would appreciate it.