Although this was an appealing conclusion, in our opinion the study was severely limited by selection bias
and confounding by underlying diseases.
Looking at therapy sequence in those who had the same type of systemic therapy, but with chemotherapy before or after surgery, you see a suggestion that giving systemic therapy first with a clinical response, which is a favorable prognostic indication, leads to a selection bias
in choosing patients for surgery.
I think the challenge for any observational study, no matter how well designed, is that without the benefit of randomization it's really tough to control for the confounder of the initial selection bias
, or confounding by indication," said Dr.
These technically-minded reports, designed primarily for research scientists and students in the field of biochemistry, explore such protein research topics as dietary protein requirements for neonatal infants, sample selection bias
from protein databases and new trends in improving the quality of milk and cheese.
However, the estimated coefficients suffer from sample selection bias
[Heckman, Econometrica, 1979].
Most studies that compare the effect of vocational and general education on labor market outcomes in the cross-section suffer from selection bias
because less able students are more likely to enroll in vocational programs.
2005), we would like to a) comment on the authors' incomplete genotoxicity review, which is inconsistent with conclusions reached by regulatory agencies; b) estimate the likely range of systemic doses and margins of exposure for farmers based on comprehensive glyphosate biomonitoring data published in 2004; and c) request further evaluation of confounding and selection bias
in their analyses for multiple myeloma.
Possible limitations of the study, include the fact that "there might have been some sort of selection bias
, either in the choice of images, or from the choice of cases," Ms.
The researchers acknowledge the possibility of selection bias
or underreporting in the study because nurses were not able to communicate with all patients in their own language or in private.
First, our study was retrospective, which implies a selection bias
The problem of selection bias
exists in the assessment of ongoing programs and non-experimental demonstrations because in such situations, without random assignment, people will select into a program based on both measurable and unmeasured or unmeasurable characteristics.
1995) attribute this selection bias
to the way in which firms are added to the COMPUSTAT database.