Species & Route of Exposure (Table 1)

A sample is a smaller subset of individuals selected from a larger population of interest. Because it is usually impossible to study every member of a population, researchers measure variables within a sample with the intention of generalizing their findings back to the broader group, ideally using a representative sample that closely resembles the population.

Sample

In the context of psychological research, how is a 'population' defined?

A developmental psychologist is studying the language acquisition skills of children with autism. She observes 50 children with autism at a local clinic. In this research context, what term best describes the entire group of 'all children with autism' that the psychologist ultimately wants to draw conclusions about?

In research, a population is the entire, often very large, group of individuals that researchers are interested in studying and drawing conclusions about. Depending on the researcher's goals, a population could be as broad as all human beings or as specific as all American teenagers, children with autism, or professional athletes.

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Animals were observed twice-daily for 28 days. Samples include blood, cerebrospinal fluid, feces, urine, bronchoalveolar lavage and mucosal swabs. These samples were collected at pre-exposure and post-exposure days and necropsy. Physical examination and imaging (radiography, PET, computed tomography) occurred 7 days prior to exposure and a week after exposure. Following the euthanization of the animals, samples from the left cranial and caudal lung lobes were processed in the media and analyzed. Right lung lobes contained in a fixative to preserve the samples. 

Animals and Procedure

A hypothesis is a specific prediction regarding the outcome of a particular scientific study. In the hypothetico-deductive method, hypotheses are derived directly from broader theories and are then tested empirically to support or reevaluate those theories. Importantly, disconfirming a hypothesis does not necessarily mean that the underlying theory from which it was derived has been disproved.

Hypothesis

A variable is any quantity, quality, or characteristic that can change or vary across different individuals, situations, or over time. In psychological research, some variables are straightforward and simple to measure, such as age or height, while others are more complex and require careful operational definition to be studied empirically.

Variable

Blair, R. V., Vaccari, M., Doyle-Meyers, L. A., Roy, C. J., Russell-Lodrigue, K., Fahlberg, M., . . . Rappaport, J. (2021). Acute respiratory distress in aged, sars-cov-2–infected african green monkeys but not rhesus macaques. The American Journal of Pathology, 191(2), 274-282. doi:10.1016/j.ajpath.2020.10.016

Acute Respiratory Distress in Aged, SARS-CoV-2–Infected African Green Monkeys but Not Rhesus Macaques

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Population

In a scientific context, deductive reasoning starts with a general premise or hypothesis and uses it to derive specific, logical conclusions about the real world. This method is employed by scientists to empirically test their hypotheses. The validity of the conclusions reached is entirely dependent on the correctness of the initial hypothesis; a correct hypothesis will lead to correct conclusions, while an incorrect one can result in conclusions that are logical but factually wrong.

Deductive Reasoning

Science is a process of continuous revision. It is fruitful to look for ways to understand the exceptions to existing theories. This can spark new ideas for hypotheses, which can allow for further understanding of the subject.

Generating Hypotheses

An example of a hypothesis that cannot be scientifically tested is the claim that 'a bird sings because it is happy.' This hypothesis is untestable because there is no objective way to measure a bird's happiness. Scientific inquiry requires phenomena to be perceivable and measurable. A testable alternative could focus on the bird's brain state, as brain activity is a measurable phenomenon.

Example of a Non-Testable Hypothesis

A hypothesis is frequently structured as an 'if-then' statement to establish a clear, testable prediction. For example, a student might form the hypothesis: 'If I study all night, then I will get a passing grade on the test.' This format explicitly connects a specific action (studying all night) with a measurable outcome (a passing grade).

Example of a Hypothesis

The process of testing specific hypotheses is fundamental to the evolution of scientific theories. When individual hypotheses are empirically evaluated, the results—whether they support or contradict the hypothesis—are used to modify and refine the broader theories from which they were derived. This iterative cycle ensures that scientific theories are dynamic and continuously updated to reflect real-world evidence.

Role of Hypothesis Testing in Theory Refinement

Scientific hypotheses originate from two primary sources. They can emerge from direct, real-world observations, where a researcher notices a pattern or potential relationship. Alternatively, they can be developed through a thorough review of existing research and established theories, identifying gaps in knowledge or proposing new connections.

Sources of Hypotheses: Observation and Prior Research

The scientific standard for establishing that one variable (X) causes another (Y) involves conducting a controlled experiment. In such an experiment, researchers intentionally change X while observing the resulting impact on Y, ensuring that other factors are held constant to confidently attribute the change in Y to the change in X.

The Scientific Standard for Establishing Causality: Controlled Experiments

A researcher is investigating the connection between the amount of time spent on a social media app and users' self-reported levels of happiness. The researcher starts with the general idea that excessive social media use might negatively affect well-being. Which of the following statements represents the most effective and scientifically testable hypothesis for an experiment on this topic?

Scientific theories and hypotheses share a fundamental logical connection framed as an 'if-then' relationship. When a researcher derives a hypothesis from a theory, they are essentially stating: 'If this specific theory is accurate, then this particular observable phenomenon should occur.' For instance, if drive theory is correct, then one would expect insects to run faster in the presence of others. This deductive structure not only clarifies the testable prediction but also serves as an effective strategy for generating compelling research questions.

The 'If-Then' Relationship between Theories and Hypotheses

Among the strongest hypotheses are those formulated to distinguish between competing scientific theories. In this scenario, researchers identify two theories that offer conflicting explanations for the same phenomenon. They then design a study and derive a single hypothesis where each theory implies a mutually exclusive outcome. Because the predictions are opposite, observing the study's results will confirm one theory's prediction while simultaneously disconfirming the alternative.

Testing Competing Theories with Hypotheses

An effective scientific hypothesis typically demonstrates three fundamental characteristics that allow it to be rigorously evaluated through empirical investigation. First, it must be testable and falsifiable, meaning it can be investigated and potentially disproven. Second, it must be logical, meaning it is informed by structured reasoning, existing theories, or prior observations. Third, it must be positive, actively asserting that a specific effect or relationship exists rather than claiming that it does not.

Characteristics of a Good Hypothesis

The scientific research process functions as a continuous cycle that starts with an overarching theory, leading to the generation of a specific hypothesis. Following this, data is collected, analyzed, and summarized to report findings. These results ultimately decide whether the initial theory is confirmed or if it requires modification. If modified, the updated theory acts as the basis for new hypotheses, thereby sustaining the ongoing cycle of scientific investigation.

The Cyclical Process of Scientific Inquiry

The James-Lange theory of emotion can serve as a basis for generating a testable hypothesis. Because the theory states that physiological arousal is a prerequisite for experiencing an emotion, a derived hypothesis could predict that a person who is completely unaware of their physiological arousal when exposed to a frightening stimulus—such as an aggressive snake—will not feel fear.

Deriving a Hypothesis from the James-Lange Theory

The scientific community's reliance on falsifiability instills significant confidence in the knowledge generated by scientific research. Because scientific claims must be testable, information typically undergoes rigorous and repeated testing before it is widely accepted.

Falsifiability and Confidence in Scientific Research

Before conducting a scientific experiment, researchers are required to formulate a specific, testable hypothesis. This explicit prediction serves as the foundation for the study, establishing exactly what relationship or effect the experiment is designed to measure and evaluate.

Requirement of a Hypothesis for Scientific Experiments

An a-theoretical hypothesis is a specific prediction that is not derived from an existing, broader theory. This often occurs when a research question is highly original, requiring researchers to formulate a hypothesis, collect data, and make observations first before a broader explanatory theory can be developed.

A-theoretical Hypothesis

Which of the following statements best describes a scientific hypothesis?

A constant is a property or quantity that does not vary across individuals or situations, such as the speed of light. Because constants do not change, they are not the focus of psychological research questions, which instead seek to understand phenomena that vary.

Constant

A quantitative variable is a type of variable that represents a measurable quantity, typically assessed by assigning a numerical value to each individual. Common examples in psychological research include a person's height, level of talkativeness, number of siblings, or score on a depression scale.

Quantitative Variable

A categorical variable is a type of variable that represents a characteristic or quality, typically assessed by assigning a descriptive category label to each individual rather than a number. Examples include a person's chosen major, nationality, occupation, or whether they are receiving psychotherapy.

Categorical Variable

An abstract construct, also referred to as an intangible variable, is a psychological characteristic or internal state that cannot be directly observed, such as self-esteem, mood, or a behavioral intention. Because psychological research relies on empirical data, researchers must develop specific methods to transform these intangible concepts into quantifiable variables so they can be measured and studied.

Abstract Construct

In experimental research, an independent variable is the presumed cause in a cause-and-effect relationship. It is the specific factor that a researcher systematically manipulates or varies to observe its impact on another variable. While experiments require manipulation, in some non-experimental or factorial designs, an independent variable may simply be measured.

Independent Variable

A dependent variable is the factor in an experiment that represents the presumed effect within a causal relationship. It is the variable that researchers measure to evaluate if it changes as a direct result of the manipulation of the independent variable.

Dependent Variable

In research, a population is the entire group of individuals that the researchers seek to draw conclusions about based on their findings from the sample. 

To illustrate how quantities and qualities can vary across individuals or situations, consider the students enrolled in a university course. The physical height of these students serves as a clear example of a variable because it differs from one individual to another. Similarly, their chosen major is also a variable, provided that the class consists of students who have declared different fields of study. These illustrations demonstrate how commonly variations occur in the world.

Example of a Variable: Height and Chosen Major

Non-experimental research is a broad category of research methodologies defined by the lack of manipulation of an independent variable. Instead of controlling factors, researchers simply measure variables as they naturally occur in real-world or laboratory settings. Although experimental research can provide strong evidence that changes in an independent variable cause differences in a dependent variable, non-experimental research generally cannot. Despite this inability to draw causal conclusions, non-experimental research is still considered extremely important and is utilized in cases where experimental research cannot be carried out.

Non-Experimental Research

The distribution of a variable describes how individual scores or values are spread across the different levels or categories of that specific variable within a dataset.

Variable Distribution

In psychology, a construct is a complex variable that defies direct observation or straightforward measurement—such as an attitude, ability, personality trait, or emotional state. These variables cannot be seen directly because they represent broad behavioral tendencies rather than isolated actions, and they involve hidden internal processes like specific feelings, thoughts, and nervous system activities. Ultimately, a construct acts as a theoretical summary that encapsulates these intricate sets of internal and external processes.

Psychological Construct

Measurement is the process of systematically assigning scores to individuals or objects so that these scores accurately represent a specific characteristic of interest. Crucially, this broad concept does not limit measurement to the use of specific physical instruments or tools; rather, it requires any methodical procedure that yields a representative score.

Measurement

An operational definition defines a psychological variable in terms of the exact procedures used to measure or observe it. Because most variables are abstract constructs that cannot be directly observed (e.g., stress), researchers must transform these conceptual definitions into concrete, quantifiable forms. For instance, stress could be operationally defined as a score on a standardized questionnaire, the concentration of cortisol in saliva, or the number of recent stressful life events experienced.

Operational Definition

A participant variable, also known as an individual difference, is a type of extraneous variable derived from the pre-existing characteristics of the individuals involved in an experiment. Examples of these variables include demographic factors such as gender, as well as personal attributes or habits like diet and writing ability. Because these individual differences are likely to have their own independent effect on the dependent variable, they must be controlled to prevent them from obscuring the effect of the experimental manipulation.

Participant Variable

Psychological measurement is the systematic empirical process of quantifying abstract variables and constructs—such as moods, self-esteem, or behavioral intentions—to facilitate scientific analysis. By utilizing standardized tools and methodologies, researchers can translate intangible psychological characteristics into objective, numerical data that can be rigorously evaluated and studied.

Psychological Measurement

The distribution of a variable describes how scores or values are spread out across the different levels or categories of that specific variable.

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