Biostats_7_Biases Flashcards
What is the term used when a test has interpreted increased survival, but has really detected the disease earlier and the clinical course of the disease has not changed … ?
Lead- time bias.
What is the solution for lead-time bias?
This can be rectified by measuring the “back-end” survival
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Adjust survival according to the severity of disease at the time of diagnosis.
What is the testing bias where the test only picks up the less aggressive forms of the disease, with long latency periods, while those with shorter latency period become symptomatic earlier and are picked up, when these are compared the survival time appears to be longer for the less aggressive form of the disease?
Length-time bias
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This is a phenomenon whereby a screening test preferentially detects less aggressive forms (with a longer-latency period) of a disease and therefore increases the apparent survival time. If a new screening test detects more non-aggressive diseases and fewer aggressive ones than the previous method of diagnosis, this may appear to increase survival.
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This can be rectified by a randomized controlled trial assigning subjects to the screening program or to no screening.
Ascertainment bias is a type of _______ bias.
Sampling bias
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These are selection biases that can occur when a study is conducted in a way that certain individuals are more likely to be selected for a study group, resulting in a nonrandomized sample of a population. This can lead to incorrect conclusions being drawn about the relationship between exposures and outcomes.
What is a referral bias?
Referral bias results when patients are sampled from specialized medical centers and therefore they do not represent the general population.
Ascertainment bias will cause a limitation in … ?
External validity
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Any ascertainment bias will limit the generalizability of the study results and interferes with the applicability of study results to the populations outside of the target population from which the sample population was drawn.
When given a choice of whether or not to participate in a study, what bias tends to occur?
Non-response bias is a sampling bias that may occur when study design allows subjects to decide whether or not to participate in the study. Health surveys conducted by a random selection of phone numbers are a prime example. The phone numbers selected are called and people are interviewed using a standardized questionnaire. There are always people who would refuse to participate in the survey. If the refusal is somehow related to their health status (e.g., they are sicker than the general population), then non-response selection bias results.
What bias results from participants lost to follow up have a different prognosis than those who complete the study?
Attrition bias and is a form of sampling bias and occurs in prospective studies. The remaining participants differ systematically from those lost to follow up.
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This bias can be mitigated with randomization.
In what type of studies does lose to follow up tend to occur?
Loss to follow-up is a form of selection bias. This occurs in cohort studies. If people from one group (exposed or unexposed) who are lost to follow-up are more likely to develop the outcome in question than those lost to follow-up from the other group, then selection bias results. A high rate of follow-up loss creates a high potential for selection bias in prospective studies.
What type of bias results when a research study mainly samples from the working population?
Healthy worker effect
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This tends to occur because workers tend to be healthier on average than the general population. Severely ill individuals do not usually work, so any sample consisting of only subjects that work is not representative of the general population.
What precaution needs to occur when a research study samples individuals who volunteer to participate in the study?
Ensure that the sample population maintains external validity because only sampling those who volunteer may have different characteristics than the general population.
When patients are enrolled on basis of ease of contact, this is called?
Convenience sampling and is a form of sampling bias.
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Sampling bias is a nonrandom sampling or treatment allocation of subjects such that the study population is not representative of target population.
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Fix with randomization
What is the bias where the cases and/or controls are selected from hospitals (bedside bias) and are less healthy and have different exposures?
Berkson bias and is a form of sampling bias.
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Selecting control subjects for a case-control study from hospitalized patients can potentially bias the results because the exposure frequency in hospitalized patients does not necessarily reflect that of the general population. This type of selection bias is called Berkson fallacy. Patients in a university hospital may have more severe illness and higher mortality rates than individuals with the same condition in a community hospital.
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Fix with randomization.
What distortion occurs with sampling bias?
Selection bias results from selection of study subjects that are not representative of the study population.
What type of bias is seen in this example:
Considering acute coronary syndrome, the healthier patients may be preferentially selected for coronary intervention, while sicker patients may instead be selected for medical therapy. Later, the outcomes due to coronary intervention appear superior to medical therapy simply because the subjects who underwent coronary intervention were healthier.
Susceptibility bias
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When a treatment regimen selected for a patient depends on the severity of the patient’s condition. This type of bias results from one disease predisposing affected individuals to another disease, and the treatment for the first disease is mistakenly interpreted as a predisposing factor for the second disease.
When patients are systematically assigned in a RCT based on the severity of disease, the study may exhibit what type of bias?
Susceptibility bias
What bias occurs when data become skewed by selective survival?
Survival bias
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A type of selection bias in which those observed as having a disease have either more severe or less severe disease than is true for all those who truly have the disease. In comparison to the true population with disease: if those with severe disease die before the moment of observation, those with less severe disease are observed, and if those with less severe disease have a resolution of their disease before the moment of observation, those with more severe disease will be observed.
What are the other terms used to describe survival bias?
Prevalence bias (Neyman bias)
How does the survival bias skew results in a “negative” manner?
If individuals with severe disease die before the moment of observation, those with less severe disease are more likely to be observed.
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For example, diabetics are more likely to die from myocardial infarction than are non-diabetics. If living patients who have sustained myocardial infarction are asked about their diabetes status, it is likely that diabetics will be under-represented because non-diabetics ‘selectively survived’ their cardiovascular events.
How does the survival bias skew results in a “positive” manner?
If individuals with less severe disease have a resolution of their disease before the moment of observation, those with more severe disease are more likely to be observed.
