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Definition
  • A systematic error in collecting or interpreting observations found in the study design
Types of Bias
Types of Bias 
Bias Description Mitigation
Accumulation Effect 
  • patients sometimes must be exposed to a risk factor for a prolonged period of time before they develop a clinically detectable result
    • e.g., patients must smoke for many pack-years before bronchogenic carcinoma develops
  • try to follow study participants for as long as is feasible 
Confounding 
  • a third factor is either positively or negatively associated with both the exposure and outcome  
  • confounders are not in the causal pathway
    • if not adjusted for can distort true association 
    • either towards or away from the null hypothesis
  • randomization 
    • ensures similar baseline characteristics between control and exposure/experimental groups
    • use intention-to-treat analysis to preserve randomization even if participants change study treatments 
  • matching
    • group similar participants into study pairs
  • stratification
    • analyze in separate subgroups determined by a potential confounder
  • restriction
    • only include groups with specific features in the sample
  • adjustment
    • can only adjust for confounders that are known and measureable
  • crossover studies 
    • subject acts as own control
Selection Bias
  • sampled population is not representative of the population researchers are trying to study 
    • due to non-random selection of study participants
    • sampling (ascertainment) bias
      • certain individuals are more or less likely to be selected for a study group, leading to incorrect conclusions
      • non-response bias
        • e.g., participants who pick up the phone may be less sick than participants who don't
      • healthy worker effect
        • samples with employed subjects only may be healthier
      • volunteer bias
        • people who volunteer for a study may be different in some fundamental way from those who do not volunteer
    • late-look bias
      • patients with severe disease are less likely to be studied, because they die or are otherwise unavailable, making a disease look less severe
        • e.g., a group of HIV+ individuals are all asymptomatic
      • also can have opposite effect
        • e.g., people with more mild disease are cured before the study takes place and only persistently sick folks are included in the study, making a disease seem more severe
    • Berkson bias
      • hospitalized study subjects are more likely to have a greater burden of illness than other possible subjects
    • attrition bias
      • those lost to follow-up may be different from those who remain in the study
  • randomization
  • include patients in multiple settings (outpatient, hospitalized)
  • study designs that are longitudinal in nature rather than cross-sectional
  • gather maximal information on participants




Measurement Bias


  • information is gathered in a way that distorts the information or misclassifies study participants 
    • interviewer bias
      • subjects in one group are interviewed in a different way than another
        • differences due to interviewing style disrepencies are falsely attributed to group differences




  • standardize data collection

Recall Bias
  • subjects with the disease are more likely to recall the exposure of interest 
    • e.g., parents of children with cancer recall exposure to a chemical
  •  reducing follow-up time in retrospective studies
Performance Bias 
  • researchers treat groups differently or subjects alter their behavior/responses due to study group awareness
    • Hawthorne effect 
      • subjects alter their behavior when they know they are being studied  
    • procedure bias
      • researcher decides assigment of treatment versus control and assigns particular patients to one group or the other nonrandomly
      • patient decides assigment of treatment versus control
  • blinding

Lead-Time Bias  

  • subjects appear to survive longer when in reality their disease was detected earlier
    • common with improved screening 
  • e.g., a cancer screening test is deemed to increase survival when in reality the disease was picked up earlier, increasing the time from detection to death
  • use mortality rate instead of survival time in screening studies
  • estimate lead time and add that to survival in unscreened group

Design Bias
  • the control group is inappropriately non-comparable to the intervention group
    • allocation bias
      • difference in the way participants are placed in control versus experimental groups
      • e.g., all zebras in control group and all lions in exposure group

 

 

  • randomization
  • matching
Cognitive Bias
  • observer bias (pygmalion effect) 
    • investigator inadvertently conveys her high expectations to subjects, who then produce the expected result
      • a "self-fulfilling prophecy"
      • golem effect is the opposite: study subjects decrease their performance to meet low expectations of investigator
  • confirmation bias
    • researcher ignores results that do not support their hypothesis
  • response bias
    • participants do not respond accurately because they are concerned about the social desirability of their responses or misinterpret the question

 

 

 


  • double blinding
  • include positive and negative results
Surveillance Bias
  • outcomes are more likely to be detected in certain groups because of increased monitoring
    • e.g., a certain skin disease being detected more often in hypertensive patients because they have more physician visits than non-hypertensive patients
    • researchers may falsely attribute hypertension to causing the skin disease




  • match participants on similar likelihood of surveillance
 
Examples of Effects that are Not Bias
  • Effect modification
    • Effect modification occurs when a third factor affects the magnitude of the relationship between the exposure and the disease
      • e.g., the increased risk of cancer in smokers is even higher among those who also drink heavily.  
      • NOT a type of bias
  • Latent period
    • The negative effects of a disease may take years to become clinically apparent
    • NOT a type of bias
  • Generalizability 
    • the ability to use results from a study to draw conclusiosn about populations different than that used in the study
    • this is most problematic for studies that evaluate only a very specific population 

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Questions (16)
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(M1.ST.15.66) Study X examined the relationship between coffee consumption and lung cancer. The authors of Study X retrospectively reviewed patients' reported coffee consumption and found that drinking greater than 6 cups of coffee per day was associated with an increased risk of developing lung cancer. However, Study X was criticized by the authors of Study Y. Study Y showed that increased coffee consumption was associated with smoking. What type of bias affected Study X, and what study design is geared to reduce the chance of that bias?

QID: 104072
1

Observer bias; double blind analysis

15%

(10/66)

2

Lead time bias; placebo

0%

(0/66)

3

Selection bias; randomization

9%

(6/66)

4

Measurement bias; blinding

0%

(0/66)

5

Confounding; randomization or crossover study

65%

(43/66)

M 1 E

Select Answer to see Preferred Response

(M1.ST.15.18) A pharmaceutical company conducts a randomized clinical trial in an attempt to show that their new anticoagulant drug, Aclotsaban, prevents more thrombotic events following total knee arthroplasty than the current standard of care. However, a significant number of patients are lost to follow-up or fail to complete treatment according to the study arm to which they were assigned. Despite this, the results for the patients who completed the course of Aclotsaban are encouraging. Which of the following techniques is most appropriate to use in order to attempt to prove the superiority of Aclotsaban?

QID: 102597
1

Per-protocol analysis

7%

(3/44)

2

Intention-to-treat analysis

16%

(7/44)

3

As-treated analysis

36%

(16/44)

4

Sub-group analysis

16%

(7/44)

5

Non-inferiority analysis

7%

(3/44)

M 1 E

Select Answer to see Preferred Response

(M1.ST.15.24) A new antihypertensive medication is studied in 3,000 Caucasian men with coronary heart disease who are over age 65. The results show benefits in terms of improved morbidity and mortality as well as a decreased rate of acute coronary events with minimal side effects. After hearing about this new medication and supporting study at a recent continuing education course, a family physician elects to prescribe this medication to a 39-year-old Hispanic female who presents with primary hypertension. After a one month trial and appropriate adjustments in the dosing, the patient's blood pressure is not well controlled by this medication. Which of the following statistical concepts could explain this patient's poor response to the medication?

QID: 104030
1

Confounding

9%

(5/57)

2

Selection bias

37%

(21/57)

3

Effect modification

0%

(0/57)

4

Generalizability

39%

(22/57)

5

Observer bias

5%

(3/57)

M 1 D

Select Answer to see Preferred Response

(M1.ST.15.21) A study is funded by the tobacco industry to examine the association between smoking and lung cancer. They design a study with a prospective cohort of 1,000 smokers between the ages of 20-30. The length of the study is five years. After the study period ends, they conclude that there is no relationship between smoking and lung cancer. Which of the following study features is the most likely reason for the failure of the study to note an association between tobacco use and cancer?

QID: 106094
1

Effect modification

6%

(9/155)

2

Latency period

53%

(82/155)

3

Pygmalion effect

6%

(10/155)

4

Late-look bias

14%

(22/155)

5

Confounding

17%

(26/155)

M 1 E

Select Answer to see Preferred Response

(M1.ST.15.4022) Two studies are reviewed for submission to an oncology journal. In Sudy A, a novel MRI technology is evaluated as a screening tool for ovarian cancer. The authors find that the mean survival time is 4 years in the control group and 10 years in the MRI-screened group. In Study B, cognitive behavioral therapy (CBT) and a novel antidepressant are used to treat patients with comorbid pancreatic cancer and major depression. Patients receiving the new drug are told that they are expected to have quick resolution of their depression, while those who do not receive the drug are not told anything about their prognosis. Which of the following describes the likely type of bias in Study A and Study B?

QID: 107007
1

Lead time bias; Pygmalion effect

47%

(15/32)

2

Lead time bias; Golem effect

6%

(2/32)

3

Latency bias; Pygmalion effect

19%

(6/32)

4

Latency Bias; Golem effect

3%

(1/32)

5

Confounding; Golem effect

6%

(2/32)

M 1 E

Select Answer to see Preferred Response

(M1.ST.14.12) A new formula for an anti-wrinkle cream is being tested for efficacy in a group of 362 healthy 40- to 60-year-old female volunteers. The marketing team randomizes the volunteers. Half receive the new formula and the other half of the volunteers receive the original formula. The mean age in the test group is 48 (95% CI 42-56), and the average age of the control group is 49 (95% CI 42-55). The volunteers are unaware of which formula they receive. The research and development team then compares before and after photographs of the volunteers following 6 weeks of at home application twice daily. For simplicity, the marketing team labels the photographs with "new formula" or "original formula." 98% of volunteers in the test group complete the study, and 97% of volunteers in the control group complete the study. The researchers conclude that there is improved wrinkle reduction with 6 weeks of use of the new formula. Which of the following potential biases most likely impacted this conclusion?

QID: 103974
1

Selection bias

8%

(3/39)

2

Hawthorn effect

8%

(3/39)

3

Observer bias

56%

(22/39)

4

Procedure bias

5%

(2/39)

5

Recall bias

3%

(1/39)

M 1 D

Select Answer to see Preferred Response

(M1.ST.14.79) A new study is investigating the effects of an experimental drug, Exerzisin, on the duration and intensity of exercise. In the treatment group participants are given daily Exerzisin at the main treatment facility and instructed to exercise as much as they would like on the facility's exercise equipment. Due to an insufficient number of exercise units at the main treatment center, the control subjects are given free access to an outside, private gym. The duration and intensity of exercise in both groups is measured with a pedometer. The perspicacious undergraduate, hired to input all the data, points out that the treatment group may be more motivated to exercise harder and longer because their exercising can be observed by the investigators. To which form of bias is he alluding?

QID: 103896
1

Selection bias

8%

(7/87)

2

Hawthorne effect

68%

(59/87)

3

Recall bias

3%

(3/87)

4

Pygmalion effect

10%

(9/87)

5

Lead time bias

1%

(1/87)

M 1 E

Select Answer to see Preferred Response

(M1.ST.14.80) A randomized controlled trial is conducted investigating the effects of different diagnostic imaging modalities on breast cancer mortality. 8,000 women are randomized to receive either conventional mammography or conventional mammography with breast MRI. The primary outcome is survival from the time of breast cancer diagnosis. The conventional mammography group has a median survival after diagnosis of 17.0 years. The MRI plus conventional mammography group has a median survival of 19.5 years. If this difference is statistically significant, which form of bias may be affecting the results?

QID: 103897
1

Selection bias

7%

(4/59)

2

Misclassification bias

2%

(1/59)

3

Lead-time bias

61%

(36/59)

4

Recall bias

3%

(2/59)

5

Because this study is a randomized controlled trial, it is free of bias

15%

(9/59)

M 1 E

Select Answer to see Preferred Response

(M1.ST.14.3) An office team is being observed by an outside agency at the request of management to make sure they are completing all their tasks appropriately. Several of the employees are nervous that they are being watched and take care to perform their jobs with extra care, more so than they would have done during a normal workday. What best describes this behavior?

QID: 104009
1

Pygmalion effect

0%

(0/26)

2

Novelty effect

0%

(0/26)

3

Observer bias

23%

(6/26)

4

Hawthorne effect

62%

(16/26)

5

Ringelmann effect

0%

(0/26)

M 2 D

Select Answer to see Preferred Response

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(M1.ST.14.83) You are reviewing the protocol for a retrospective case-control study investigating risk factors for mesothelioma among retired factory workers. 100 cases of mesothelioma and 100 age and sex matched controls are to be recruited and interviewed about their exposure to industrial grade fiberglass by blinded interviewers. The investigators' primary hypothesis is that cases of mesothelioma will be more likely to have been exposed to industrial grade fiberglass. The design of this study is most concerning for which type of bias?

QID: 103900
1

Interviewer bias

6%

(2/35)

2

Recall bias

69%

(24/35)

3

Observer bias

14%

(5/35)

4

Lead-time bias

0%

(0/35)

5

This study design is free of potential bias

6%

(2/35)

M 1 D

Select Answer to see Preferred Response

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