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Old 08-07-2012
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Stats What would contribute to selection bias in this study?

In a study to examine the effect of a new drug to treat depression, patients with depression receive either the new drug or a placebo for 1 year. What would contribute to selection bias in this study?

a) Information is collected every 2 months from the intervention group and at the end of the study from the placebo group
b) Patients are recruited from a psychiatric in-patient facility
c) Patients report depression symptoms using nonstandard descriptors
d) Side effects, or the lack thereof, make group assignment obvious to patients
e) The intervention group has a history of longer duration of depression than the placebo group
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a) Information is collected every 2 months from the intervention group and at the end of the study from the placebo group-Expermenter expectancy/observer bias
b) Patients are recruited from a psychiatric in-patient facility-Sampling bias(Berkson bias)
c) Patients report depression symptoms using nonstandard descriptors-recall bias
d) Side effects, or the lack thereof, make group assignment obvious to patients
e) The intervention group has a history of longer duration of depression than the placebo group-Design bias.
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is it b???
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Old 08-07-2012
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i choose b. select pt from clinic is clearly not random assignment.
can u give explanation for each choice?
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B is my answer
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B selection or sampling bias
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Originally Posted by Kabutar111 View Post
B selection or sampling bias
selection bias is one thing and sampling/design bias is another thing.

sampling/design bias is who we recruit to participate in the study - is the group representative for the population.
selection bias is what we do with those people we already recruited, e.g. how we assign them to control/placebo group or treatment group.
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Default correct answer :)

The answer is e) The intervention group has a history of longer duration of depression than the placebo group

Selection bias occurs when groups differ in important characteristics other than the exposure of interest. In this situation, patients with a shorter history can have less severe or less complicated symptoms, or can be more responsive to some interventions.

As for A) - Assessing outcome in different ways in different groups can result in measurement bias, not selection bias.

As for B) - Patients recruited from an in-patient facility are not representative of all patients with depression. For example, they can have more serious depression or more management issues. This would result in sampling bias, not selection bias.

As for C) - Nonstandard or ill-defined measurement procedures can result in unreliable assessment of outcome, not selection bias.

As for D) - Patient awareness of group assignment reflects failure of masking, which can lead to the differential assessment of outcome or systematic differences in behavior during the course of treatment. This would result in measurement bias, not selection bias.
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Quote:
Originally Posted by Casandra View Post
selection bias is one thing and sampling/design bias is another thing.

sampling/design bias is who we recruit to participate in the study - is the group representative for the population.
selection bias is what we do with those people we already recruited, e.g. how we assign them to control/placebo group or treatment group.
Although I do not disagree as I do remember reading diff of them but
You can see the attached image from Kaplan as it says end result is sample not representative of general population What would contribute to selection bias in this study?-imageuploadedbytapatalk1344417321.385508.jpg
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Quote:
Originally Posted by Casandra View Post
selection bias is one thing and sampling/design bias is another thing.

sampling/design bias is who we recruit to participate in the study - is the group representative for the population.
selection bias is what we do with those people we already recruited, e.g. how we assign them to control/placebo group or treatment group.
According to kaplan:

Selection/Sampling bias is same ie sample selected is not represented of the population

Design Bias is having a non comparable group

Dont you think you have interchanged these terms in your answer explanation as well????? please correct me if i am wrong
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Quote:
Originally Posted by offpiste View Post
According to kaplan:

Selection/Sampling bias is same ie sample selected is not represented of the population

Design Bias is having a non comparable group

Dont you think you have interchanged these terms in your answer explanation as well????? please correct me if i am wrong
yeah, sorry for confusing you

in my post to Kabutar it should say: selection bias is one thing and design bias is another thing. and accordingly afterwards

nothing has been interchanged in the post where I wrote the explanations to all the options. I double checked it
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Quote:
Originally Posted by Casandra View Post
yeah, sorry for confusing you

in my post to Kabutar it should say: selection bias is one thing and design bias is another thing. and accordingly afterwards

nothing has been interchanged in the post where I wrote the explanations to all the options. I double checked it
I don't understand design bias is diff I know so. I said selection and sampling not anything about design selection is not design
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Berksonian's Bias
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