S. S. Stevens was a psychologist who introduced the widely used framework of the four levels of measurement: nominal, ordinal, interval, and ratio. In 1946, he proposed that scores assigned to individuals can communicate varying amounts of quantitative information about a given variable. His conceptualization emphasizes the generality of measurement and serves as a fundamental guide for determining which statistical procedures are appropriate for different types of data.

S. S. Stevens

The nominal level of measurement is the lowest level of measurement, used for categorical variables. It involves assigning scores that function simply as category labels, communicating only whether any two individuals are the same or different regarding the measured variable. Responses are merely categorized without implying any inherent order or ranking among them, such as classifying individuals by their marital status or favorite color. For this level of measurement, the only available measure of central tendency is the mode.

Nominal Level of Measurement

The ordinal level of measurement involves assigning scores that represent the rank order of individuals on a given variable. This level communicates not only whether individuals differ, but also whether one is higher or lower than another, such as ranking runners in a race or rating satisfaction from least to most. However, ordinal scales do not assume that the differences between adjacent scale values are equal; they only provide the sequence or order of responses. For ordinal-level measurement, the median or mode can be used as indicators of central tendency.

Ordinal Level of Measurement

The interval level of measurement involves assigning scores on a numerical scale where the intervals between values have the same consistent interpretation throughout. For instance, the difference between $$30$$ and $$40$$ degrees Fahrenheit represents the same amount of temperature change as between $$80$$ and $$90$$ degrees. In psychology, the intelligence quotient (IQ) is measured at the interval level; the difference between IQ scores of $$80$$ and $$100$$ is the same as between $$120$$ and $$140$$. However, interval scales lack a true zero point that indicates the complete absence of the measured quantity, meaning it is not meaningful to compute ratios of these scores. Furthermore, interval-level measurement permits any indicators of central tendency to be computed, including the mean, median, or mode.

Interval Level of Measurement

The ratio level of measurement is the highest level of measurement, encompassing all properties of nominal, ordinal, and interval scales, with the addition of a true zero point. This true zero represents the complete absence of the measured quantity, such as zero Kelvin indicating no kinetic energy, or zero money indicating no money. Because of the true zero, it allows for meaningful statements about the ratios of scores. For example, while one cannot say someone with an IQ of $$140$$ is twice as intelligent as someone with an IQ of $$70$$ because IQ is an interval scale, one can say that someone with $$6$$ siblings has twice as many as someone with $$3$$ because the number of siblings is measured at the ratio level. Additionally, ratio-level measurement permits any indicators of central tendency to be computed, including the mean, median, or mode.

Ratio Level of Measurement

Based on S. S. Stevens's concept of levels of measurement, what is one of the main practical reasons for categorizing variables into nominal, ordinal, interval, or ratio levels?

The concept of levels of measurement, proposed by S. S. Stevens, categorizes the assignment of scores to individuals based on the type of quantitative information conveyed about a variable. There are four distinct levels: nominal, ordinal, interval, and ratio. These levels are significant for two main reasons. First, they emphasize the broad generality of measurement itself, illustrating that even categorizing or ranking individuals qualifies as measurement provided it represents a specific characteristic of the individuals. Second, they serve as a practical guide for determining which statistical procedures are appropriate to use with the collected data and what conclusions can be drawn.

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The process of collecting information related to the research questions.

Data Collection Methods

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

https://kpu.pressbooks.pub/psychmethods4e/

KPU Research Methods in Psychology - 4th American Edition

Research methods used by researchers to collect qualitative or quantitative data in studies.

Data collection methods in UX research

Levels of Measurement

Measures range and scope of an actor's network, which includes both informal relations to other actors as well as formal memberships to clubs, organizations, groups, etc.  

Extensivity

Measures frequency of interaction and degree of involvement that actors have to other actors and institutions.

Intensivity

Psychometrics is the specific field of psychological measurement. It involves applying systematic procedures to assign scores to individuals, ensuring that these scores accurately represent psychological characteristics of interest, such as cognitive abilities or clinical symptoms.

Psychometrics

An example of scientific measurement in physics is calculating the potential energy of an object within Earth's gravitational field. This process requires finding the object's mass and height, and multiplying these values by the gravitational acceleration of Earth ($$9.8 	ext{ m/s}^2$$). The outcome is a score representing the object's potential energy, illustrating how psychological measurement shares the same systematic approach to assigning scores as the physical sciences.

Measuring Potential Energy

Measurement is a concept familiar in daily life, where individuals routinely use tools to assign scores that represent specific physical characteristics. Common examples include stepping onto a bathroom scale to determine one's weight, or using a meat thermometer to check the internal temperature of a roasting turkey. These everyday activities illustrate the foundational principle of measurement: employing a systematic procedure to obtain a numerical score that accurately represents a characteristic of interest.

Everyday Measurement

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

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

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

To maximize the reliability and validity of a psychological measure, researchers must implement it under optimal, standardized conditions. Ideally, participants should be tested in quiet, distraction-free environments. While administering measures to groups of participants simultaneously is highly efficient, it can introduce distractions that degrade data quality. Therefore, researchers should rely on prior studies as a benchmark to determine whether group testing is appropriate for their specific instrument.

Implementing a Psychological Measure

Participant reactivity occurs when individuals alter their behavior or responses simply because they are aware they are being measured, which can significantly compromise the reliability and validity of the resulting data. While some disagreeable participants might intentionally disrupt the research, reactivity more frequently manifests as agreeable participants adjusting their answers to match what they perceive the researcher expects.

Participant Reactivity

After administering a psychological measure to a sample and collecting the resulting scores, researchers must thoroughly evaluate the tool for both reliability and validity. Even if a measurement tool has a strong track record in previous studies, it is essential to verify that it functioned effectively with the current participants and under the specific conditions of the new study. Regardless of expectations, this evaluation process generates new evidence regarding the measure's psychometric properties, which should be documented and added to the broader scientific literature.

Evaluating the Measure

The process of measuring a psychological construct for a research study generally involves four broad steps: conceptually defining the construct, operationally defining the construct, implementing the measure, and evaluating the measure.

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