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Difficulty: MediumBias and Confounding in Research

A multi-center prospective cohort study investigates the efficacy of a novel intensive behavioral therapy program for adults with severe obesity. Participants are assigned to either the intensive therapy program, which requires three in-person counseling sessions per week, or a standard care program, which involves one visit every three months. The primary outcome is achieving greater than a 15%15\% reduction in baseline body mass index (BMI) at 2424 months. At the conclusion of the study, the investigators report that the intensive therapy group is significantly more likely to achieve the primary outcome compared to the standard care group. However, an independent methodological review reveals that 45%45\% of the patients in the intensive therapy program were lost to follow-up, compared to only 5%5\% in the standard care group. Furthermore, the patients who withdrew from the intensive program had significantly higher baseline BMI and a higher prevalence of psychiatric comorbidities than those who remained in the study. Which of the following best describes the most likely threat to the internal validity of this study's findings?

  1. Selective loss of participants leading to a non-representative remaining sampleAnswer
  2. B
    An external variable independently influencing both the therapy assignment and the weight loss outcome
  3. C
    Participants modifying their dietary behavior simply because they are aware of being monitored
  4. D
    Systematic differences in how accurately subjects remember and report their past dietary habits
  5. E
    Researchers subconsciously recording body measurements more favorably for the treatment group

Answer

Selective loss of participants leading to a non-representative remaining sample
The correct answer accurately describes attrition bias, which is a subtype of selection bias. Attrition bias occurs in prospective cohorts and randomized controlled trials when the loss to follow-up is non-random and disproportionately affects one group. In this scenario, the intensive therapy group experienced a 45%45\% dropout rate, and those who dropped out had more severe disease (higher baseline BMI, more psychiatric comorbidities). As a result, the patients who completed the intensive therapy were systematically healthier or more motivated than the original group, artificially inflating the apparent efficacy of the intervention when compared to standard care.

Step-by-Step Solution

1
Analyze the study design and the identified flaw in the stem.
The study is a prospective cohort with a 45%45\% dropout rate in the treatment group versus 5%5\% in the control group. Dropouts had higher BMI and more comorbidities.
Identifying the specific methodology flaw is required to determine the type of bias.
2
Correlate the flaw with specific epidemiological biases.
Unequal loss to follow-up that results in a systematic difference between the remaining groups is known as attrition bias, a subtype of selection bias.
Matching the scenario to the textbook definition distinguishes it from confounding or measurement errors.
3
Select the option that defines attrition bias.
The option describing 'selective loss of participants leading to a non-representative remaining sample' perfectly defines attrition bias.
This mechanism directly explains why the internal validity is compromised.

Key Concept

Attrition bias is a form of selection bias that occurs in prospective studies when loss to follow-up is unequal between groups and not entirely random, leaving remaining groups that are systematically different from the original cohorts.
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Topics

Bias and Confounding in Research

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