Recall bias usually occurs in studies which involve self-reporting, such as case-control studies and retrospective cohort studies.

Types of Studies where Recall bias happens

- Can potentially exaggerate the relation between a potential risk factor and the disease.
- Might not remove factors which may not be correalted

Major Problems with recall bias

Wash out period:
            a certain fixed time period that must elapse between the subject's first observation and their subsequent observation of the same event.

How to Account for recall bias

A patient being treated for ADHD might remember certain large events in time of panic or neglect small gradual changes

A trial of cancer patients not sharing family history

Example of recall bias

Take Tobacco case as an example, the interviewer must have known whether they had cancer or not, which is likely to affect their recollection. 

Another example of recall bias

Recall bias is a error that occurs when participants do not remember previous events or experiences accurately or omit details: the accuracy and volume of memories may be influenced by subsequent events and experiences.

University of Michigan - Ann Arbor

- Recall bias
- Selection bias
- Collider bias
- Omitted variable bias

Types of Bias

Depending on the study and hypothesis, a cohort study is not the best design that can be used. However, the biases included in this list can be avoided with good experimental design.

 - Expensive to conduct 
 - Prospective cohort study is often time-consuming
 - Inefficient for rare outcomes 
 - Selection bias 
 - Recall bias

Weaknesses of Cohort Studies

Catalogue of Bias Collaboration, Nunan D, Bankhead C, Aronson JK. Selection bias. Catalogue Of Bias 2017: [www.catalogofbias.org/biases/selection-bias/](https://www.catalogofbias.org/biases/selection-bias/) 

Catalog of Bias

Pearl, J., & Mackenzie, D. (2018). $\mathit{The \ Book \ of \ Why: \ The \ New \ Science \ of \ Cause \ and \ Effect.}$ Basic Books.

The Book of Why

Recall Bias

When a single node is caused by two other nodes we encounter something called "collider-bias" where one condition explains-away another. 

Since A and C both effect the outcome of B, by conditioning on B, A and C become **dependent**.

$A \rightarrow B \leftarrow C$: The Collider

This type of bias is also known as "Confounded." It happens when good controls or confounders are not included in the regression model.

Omitted variable bias

Sampling bias is a systematic error that occurs when the method used to select participants results in a sample that fails to accurately represent the broader target population. Because the selected individuals differ in important ways from the overall group, any statistical findings derived from this unrepresentative sample will likely be inaccurate and cannot be safely generalized.

Sampling Bias

Time consumption is an issue in prospective cohort studies as the investigator must reach out to the participants over a time frame whereas in retrospective studies, the information is already recorded. If the follow-up period is extremely long for a study, this could result in a series of issues. 
 - The study participants may outlive the investigator.
 - Non-response bias - Participants may begin to leave the study or may not be contactable; Participants who left the study may have different outcomes than those who stayed, resulting in a loss of data

Weakness of Time Frame in Prospective Cohort Studies

Rare outcomes would require an incredibly large study population. They can also have a long latency period, which could result in having a  follow-up period, which introduces its own problems. 

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