A cohort study is conducted to observe multiple groups of people over a period of time for comparison to see the potential effect of a phenomenon (possible influential factor) that the investigators are interested in.  The cohort studies are done either prospectively or retrospectively.  

Cohort Studies

If substance A leads to biological reaction B, then in general (but not 100%), a larger dose of A leads to the stronger reaction B. 


Dose-response effect

Case-control studies are observational studies where subjects' previous experiences are used as evidence to assess how those who had experience with the specified condition varied from those who did not and why.

Case-control studies

-Observational studies can be conducted in the natural habitat of the target population as opposed to an artificial, controlled setting in laboratory
-Observational studies maintain "purity" in that there are few issues of ethics or feasibility due to no need for intervention being applied. For example, if investigating smokers and health, we cannot make non-smokers start smoking for the experiment, but we can find people who are already smokers. 

Advantages of Observational Studies 

An observational study, as supposed to an experimental study is a type of study that investigators observe the effect of an intervention without assigning it to the subjects at random.  That is, the subjects choose their own treatment/ intervention.  

Observational studies may prove to be less costly that RCT's, given that the investigator need not actively change the behavior of the subjects


University of Michigan - Ann Arbor

Quantitative research is an empirical methodology wherein researchers start with a specific research question, collect numerical data from a sizable sample of participants, and analyze it using statistical techniques. As the most widely used approach in psychology, its primary objective is to deduce generalizable conclusions about a larger population.

Quantitative Research

Pearl notes two ways of eliminating unmeasurable confounders, in the context of:

- RCT
- Observational studies

In each of these methods, we can use the front-door criterion, back-door criterion, or if necessary, do-calculus.

Deconfounding/Eliminating Unmeasurable Confounder Variables

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

The Book of Why

Observational Studies

A Randomized Controlled Trial (RCT) or a Controlled Experiment is a method to eliminate unmeasurable confounders effect on causal inference by implementing an intervention on a randomly selected group of individuals (treatment/intervened group) while keeping other individuals unchanged (controlled group, placebo, not-intervened group) to observe the changes in outcomes due to the intervention effect. The RCTs are intended to "disarm" confounders (confounding variables).

Randomized Controlled Trial (RCT) = Controlled Experiment

Qualitative researchers often criticize quantitative methods for overlooking the rich complexity of human behavior and experience. They argue that quantitative approaches tend to oversimplify phenomena by focusing only on answering simple questions about easily quantifiable variables. However, quantitative researchers acknowledge this oversimplification, utilizing it as a deliberate strategy to uncover broad, general principles of human behavior.

Criticisms of Quantitative Research

Group research is the most common quantitative research approach in psychology, characterized by studying a large number of participants and combining their data to draw general conclusions about human behavior. In this methodology, researchers primarily analyze and interpret behavior by examining aggregated statistical metrics across the entire sample, such as group means and standard deviations, rather than focusing on the detailed behavior of individual subjects.

Group Research

The primary strength of quantitative research lies in its ability to provide precise answers to specific, tightly focused research questions and to draw broad, general conclusions about human behavior. By employing statistical analysis on large samples, this methodological approach allows researchers to uncover general behavioral trends, such as the overall tendency of individuals to obey authority figures or comparing traits across demographic groups.

Strengths of Quantitative Research

Single-subject research, frequently referred to as small-$$n$$ design (where $$n$$ represents the statistical symbol for sample size), is a quantitative methodology that involves studying a small number of participants and focusing closely on each individual's behavior. Despite its name, this approach typically investigates between two and ten participants rather than just one. Serving as a crucial alternative to group research, particularly in applied areas of psychology, single-subject research seeks to comprehend objective behavior by utilizing strict experimental manipulation and control to collect highly structured data that is analyzed using quantitative techniques.

Single-Subject Research

Despite its ability to yield precise answers and general conclusions, quantitative research is inherently limited in its capacity to generate novel or unexplored research questions. Furthermore, it is not well-suited for providing rich, detailed descriptions of specific groups' behaviors within particular contexts, nor does it effectively convey the subjective, lived experience of being a member of a given group.

Weaknesses of Quantitative Research

Qualitative and quantitative research differ significantly in their methodology and goals. Quantitative research generally starts with a focused hypothesis, gathering numerical data from large samples to draw general population conclusions using statistical techniques. In contrast, qualitative research begins with a broader, less focused research question and collects large amounts of unfiltered, nonnumerical data from a small number of individuals. Qualitative analysis relies on nonstatistical techniques to deeply understand the detailed experiences of participants rather than generating broad statistical generalizations.

Comparison of Qualitative and Quantitative Research

Survey research is a flexible methodology that incorporates both quantitative and qualitative approaches, defined by two primary characteristics. First, it relies on self-report measures, where researchers ask participants to directly report on their thoughts, feelings, and behaviors using tools like questionnaires or interviews. Second, it emphasizes rigorous sampling techniques, typically favoring large random samples to acquire accurate estimates of population characteristics. This makes survey research one of the few approaches in psychology that routinely relies on random sampling.

Characteristics of Survey Research

Which of the following statements best describes the methodology and primary objective of quantitative research in psychology?

For Causal Nodes X,Y:
X and Y will be deconfounded if we block every backdoor, because such paths allow for spurious correlations. 

When eliminating backdoors, we additionally need to ensure that anything closed off from X is not a descendent of X.

Back-Door Criterion

Pearl introduces an axiomatic system containing three rules to remove do operators in Chapter 7. 

Rule 1. Irrelevance
Rule 2. Conditional equivalence of *do(x)* and *see(x)*
Rule 3. Lack of causal paths

Generalizing Do-Calculus

When the causal effect of a variable $X$ on $Y$ is mediated by a variable $M$, which is shielded from a confounding set $C$, if $C$ is *unmeasurable*, we cannot block the back-door $X \leftarrow C \rightarrow Y$. So, we use the front-door $X \rightarrow M \rightarrow Y$. 

Front-Door Criterion (Unmeasurable confounder of a mediated path)

$\ \ \ \ \ \ \ \ \ \ \ C$

$\ \ \ \ \swarrow \ \ \ \ \ \ \ \ \searrow$

$X \rightarrow M \rightarrow Y$

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