Ch. 9: Bias

True or False: Individuals at an elevated risk for a given medical condition are as likely to volunteer to take part in a RCT to prevent that condition as are individuals at moderate risk for that condition.This is an example of selection bias in a RCT.

False

Same scenario as above: A prospective, multi-site cohort study enrolled 12,000 adult women between 24 - 45 years of age to examine the relationship between alcohol consumption and risk of breast cancer. The cohort study involved 10 years of follow-up with visits scheduled once a year. Of the 12,000 women initially enrolled, 3,458 women withdrew from the study at some point over the 10-year follow-up period. At the end of the study, the investigators found a CIR = 1.1 (95% Confidence interval 0.84, 1.90) for the relationship between alcohol consumption and breast cancer risk - indicating In order to understand how they could have obtained this result, the investigators compared the baseline data for those who remained in the study (completed all follow-up visits) and those withdrew from the study. This comparison revealed that women who had dropped out of the study were more likely to report having a close female relative who had breast cancer (60%) and being regular drinkers (84%) compared with women who remained in the study for the entire follow-up period (37% with family history of breast cancer and 59% regular drinkers).

How might these losses have influenced study results (select ALL that apply): - This study illustrates a high likelihood of selection bias due to loss to follow-up - Those lost to follow-up tended to be women with a family history of breast cancer as well as regular drinkers. Family history of breast cancer is a very strong risk factor for breast cancer, so if they had been included in the study results, the findings may have changed (i.e., an association between alcohol consumption and breast cancer may have been found) - It's unknown if the loss to follow-up was among drinkers or those with a family history of breast cancer; so it's unclear if they had been included in the study results, whether or not the findings may have changed

True or False: An investigator conducting a population based cross-sectional study finds that individuals who answered the landline-based telephone survey tended to be older than individuals who answered a cellphone-based telephone survey. The above example is an illustration of selection bias in a cross-sectional study.

True

True or False: The following is an example of selection bias: The participants who were enrolled in the study were the easiest to reach, and not representative of the participants who were more difficult to reach.

True

True or False: A researchers recruit cases and controls based on likelihood of exposure status.This is an example of selection bias in a case-control study.

True

In a large, hospital-based case-control study of smoking and lung cancer was conducted across 5 major cancer speciality hospitals in the US. Eligible case participants were individuals newly diagnosed with lung cancer. Eligible control participants were individuals newly diagnosed with noncancerous chronic pulmonary diseases (i.e. chronic bronchitis and emphysema). The investigators found that the association between smoking and lung cancer was relatively weak (OR = 1.1, p<0.05). This finding was surprising as a later case-control study of lung cancer and smoking found an OR = 3. Which of the following terms correctly identifies the direction of bias when comparing the measure of association obtained in the hospital-based case-control study compared with the measure of association reported in the later case control study ? (Select ALL that apply)

- Negative bias - Bias toward the null value

A cohort study was conducted to examine whether exposure to certain types of pesticides during early childhood increased the risk of developing neurological problems in later adulthood. The investigators obtained a CIR = 2.6 for the relationship between early childhood pesticide exposure and neurological problems in adulthood. However, upon re-analysis of the data, it appears that there is no relationship between early childhood pesticide exposure and neurological problems in adulthood (CIR = 1). Which of the following terms correctly identifies the direction of bias when comparing the original CIR=2.6 compared with the CIR=1.0 obtained when the data were re-analyzed? (Select ALL that apply)

- positive bias - bias away from the null value

True or False: An epidemiologist finds that individuals of one race or ethnicity are more likely to withdraw from a cohort study than those of any other race or ethnicity.This is an example of selection bias in a cohort study.

True

What is the definition of selection bias?

A type of bias that occurs when participants are recruited into a study in such a way that the study sample is not representative of the target population

A prospective, multi-site cohort study enrolled 12,000 adult women between 24 - 45 years of age to examine the relationship between alcohol consumption and risk of breast cancer. The cohort study involved 10 years of follow-up with visits scheduled once a year. Of the 12,000 women initially enrolled, 3,458 women withdrew from the study at some point over the 10-year follow-up period. At the end of the study, the investigators found a CIR = 1.1 (95% Confidence interval 0.84, 1.90) for the relationship between alcohol consumption and breast cancer risk - indicating In order to understand how they could have obtained this result, the investigators compared the baseline data for those who remained in the study (completed all follow-up visits) and those withdrew from the study. This comparison revealed that women who had dropped out of the study were more likely to report having a close female relative who had breast cancer (60%) and being regular drinkers (84%) compared with women who remained in the study for the entire follow-up period (37% with family history of breast cancer and 59% regular drinkers).

The scenario is an example of: selection bias

If a study has good internal validity, what does that mean?

The study findings accurately reflect the true measure of association between the exposure and outcome.

If a study has good external validity, what does that mean?

The study findings are generalizable to the underlying or target population.

A cross-sectional study was conducted to examine the relationship between regular exercise and obesity in children 8-12 years old. Based on prior findings from studies of adolescents, the investigators hypothesized that lack of regular exercise was also going to be associated with obesity in young children. They conducted a study with a sample of n=24 children in residing in Ohio. Once the data were analyzed, the investigators found that there was no statistically significant association between exercise habits and obesity (OR = 1.1, p = 0.11). The most likely explanation for the results obtained here is:

random error