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  #1  
Old 12-06-2011
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Stats Two Bias Questions

case 1 0f 2


A doctors wants to analyze the effectiveness of using mammogram to screen for breast cancer. In her analysis, she compares the disease-free survival rate in patients screened using mammogram to the disease-free survival rate in patients diagnosed with breast ca prior to screening with mammogram . She concludes that screening is effective with mammogram , because the disease-free survival rate is higher in screened patients compared to unscreened patients.An oncologist reviews these findings and the epidemiologist's conclusions. He argues that her conclusions are incorrect, because improved survival rates following screening programs are the result of treating less aggressive tumors, not because of the screening program itself. The oncologist is arguing what type of bias has occurred in the epidemiologist's findings?
A. Lead-time bias
B. Length-time bias
C. Overdiagnosis bias
D. Recall bias
E. Selection bias



case 2 0f 2

A doctors is attempting to characterize the link between smoking and bladder ca in a large population. He plans to use a large paper-based questionnaire that will be randomly mass mailed to approximately 10,000 individuals. In the questionnaire, there are numerous questions on alcohol and smoking use, current health issues, and socioeconomic indicators. The questionnaire will need to be mailed back to the epidemiologist and he will provide all individuals with a self-addressed stamped envelope to facilitate this process. What type of bias is this study most likely to suffer from?

A. Compliance bias
B. Lead-time bias
C. Overdiagnosis bias
D. Publication bias
E. Selection bias
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  #2  
Old 12-07-2011
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1)lead time
2)compliance?
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Old 12-07-2011
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1.-B. Length-time bias
2.-E. Selection bias
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Old 12-07-2011
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case 1 0f 2

E. Selection bias



case 2 0f 2

A. Compliance bias
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Old 12-07-2011
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Quote:
Originally Posted by bebix View Post
1.-B. Length-time bias
2.-E. Selection bias
yes bebix.. thats the answer that is given.. But can u please explain why its not Lead time bias and compliance bias
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Old 12-07-2011
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Case 1:
- difference between lead time and length time bias:

" the survival rate for screened patients was entirely owing to lead-time bias (earlier diagnosis) and length-time bias (less severe cases diagnosed) associated with screening test detection, and not to any benefit from the screening strategy itself."

Case 2:

- the Probability of not responding the questionnaire is greater among patients who smoke/drink... = Selection bias
- Compliance bias: occurs when differences in subject adherence to the planned treatment regimen or intervention affect the study outcomes.
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Old 12-07-2011
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1. lead time bias
2. selection bias
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Old 12-18-2011
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Taken from wikipedia,

Length time bias can affect data on screening tests for cancer.

Faster-growing tumors generally have a shorter asymptomatic phase than slower-growing tumours, and so are less likely to be detected. However, faster-growing tumors are also often associated with a poorer prognosis. Slower-growing tumors are hence likely to be over-represented in screening tests. This can mean screening tests are erroneously associated with improved survival, even if they have no actual effect on prognosis
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