[Note: The bulk of this glossary was written in 2003 by Patricia A. Lauer, a principal researcher at Mid-continent Research for Education and Learning.]

bias - any effect that is introduced into an experiment or research study that may influence the outcome based on anything other than the variables involved (e.g. expectations, the use of inappropriate statistics)

aggregated data - data for which individual scores on a measure have been combined into a single group summary score

Example:
In education research, it is common to aggregate individual student scores on an achievement test into a mean score for each school. Researchers then use the aggregate school achievement score for data analyses. Aggregating data reduces the sample size (e.g., from 5,000 students to 10 schools.) Aggregating data also obscures differences among individual scores.

case study - a data-collection method in which a single person, entity or phenomenon is studied in depth over a sustained period of time and through a variety of data

Example:
A researcher conducts a yearlong case study of a school district that was awarded a grant to improve teacher quality. The researcher documents the processes used to implement the grant, interviews teachers and administrators, observes staff development and measures student achievement before and after the grant was awarded.

comparative descriptive study - a research study in which data are collected to describe and compare two or more groups of participants or entities

Example:
A researcher identifies high-poverty schools in the state that have either high or low student achievement. The researcher describes the alignment or match between each school's curriculum and state standard, and compares the high- versus low-achieving schools to determine whether the degree of alignment is different.

control - the strategy used in scientific research to regulate the effects of variables that are not intended to influence the results or conclusions

Example:
A researcher conducts a study of two different teacher preparation courses on how to teach mathematics. The researcher controls for differences among preservice students by randomly assigning the students to one of the two courses. The researcher controls for differences among course instructors by having a single instructor teach both courses.

control group - the group that does not receive the experimental treatment and is therefore used as a reference or comparison group for effects of the experimental treatment.

correlational research/study - nonexperimental research in which data are collected to determine the relationship between them

Example:
In School District X, a researcher collects data on beginning teachers' scores on the state licensing test (variable 1) and data on the achievement gain s of each teacher's students (variable 2). The researcher then uses correlational statistics to measure the association between the two variables.

dependent variable - the variable measured in a study - the "outcome." In experimental research, the dependent variable is affected by the independent variable. In correlational research, the dependent variable is associated with one or more other variables

Examples:
In an experimental research study, a researcher randomly assigns teachers in a large elementary school to receive one of three types of professional development: a class on instructional strategies, a training program on how to increase student motivation or a teacher discussion group. The researcher measures the differences in achievement gains among the students of the three teachers. The dependent variable is student achievement gains.

For a correlational research study, a researcher collects data on beginning teachers' scores on the state licensing test and data on the achievement gains of each teacher's students (variable 2). The researcher then uses the association between the two variables to estimate student achievement gains. The dependent variable is student achievement gains.

descriptive study - a research study that has the goal of describing what, how or why something is happening

discriminant analysis - a type of statistical analysis in which there is only one dependent variable but multiple independent variables

effect size - the degree to which a practice, program or policy has an effect based on research results, measured in units of standard deviation

Example:
A researcher finds an effect size of d = .5 for the effect of an after-school tutoring program on reading achievement. This means (provided that the research study is valid) that the average student who participates in the tutoring program will achieve one-half standard deviation above the average student who does not participate. If the standard deviation is eight points, then the effect size translates into four additional points, which will increase a student's ranking on the test.

empirical research/empirical studies - research that seeks systematic information about something that can be observed in the real world or in a laboratory

ethnography - a data-collection method in which information is collected about an intact group of individuals in their natural setting, primarily through observations

Example:
A researcher studies the challenges that face three beginning teachers at one elementary school. The researcher observes and documents the teachers in their classrooms, on the playground, in the teachers' lounge, at staff meetings, at parent conferences and in staff development sessions.

experimental study (experimental research) - a research study that has the goal of determining whether something causes an effect

external validity - the degree to which results from a study can be generalized to other participants, settings, treatments and measures

hypothesis(es) - a statement about the researcher's expectations concerning the results of a study

Examples:
A new standards-based mathematics curriculum will benefit elementary students at all grade levels.

A new standards-based mathematics curriculum will have different effects on elementary students depending on grade level.

independent variable - in experimental research, the variable that the researcher varies or manipulates to determine whether it has an effect on the dependent variable

Example:
As part of an experiment, a researcher randomly assigns teachers in a large elementary school to receive one of three types of professional development: a class on instructional strategies, a training program on how to increase student motivation or a teacher discussion group. The researcher measures the differences in achievement gains among the students of the three teachers. The independent variable is professional development, and it has three different values.

longitudinal data - data collected from the same participants at different points in time. The purpose is to make conclusions about individual change over time

Example:
A researcher studies the mathematics achievement of students who were taught a new standards-based mathematics curriculum when they were in 6th grade. The researcher compares their performances in mathematics achievement in grades 7, 8 and 9 to the performance of another group of students at each of those grade levels who were not taught the new curriculum in 6th grade. The purpose of the research is to determine whether change in mathematics performance over time is related to the type of 6th-grade mathematics curriculum.

meta-analysis - a comprehensive, systematic and quantitative review of past empirical research studies on a specific topic; most meta-analyses examine only quantitative studies; effect-size statistics are calculated to produce an overall conclusion about the various studies on the topic

Example:
A researcher conducts a meta-analysis of computer-assisted instruction in reading. The researcher examines 40 studies and calculates an overall effect size of d = .25, indicating a small positive effect of computer-assisted instruction on reading achievement.

methodology - the methods, procedures, rules, and postulates employed in accordance with a particular discipline

peer-reviewed - a research study that has been critiqued by other researchers prior to publication or presentation at a research conference

practical significance - the degree to which a practice, program or policy has enough of an effect to justify its adoption

proxy - a measure used to approximate the data sought when it is difficult to get a more precise measure due to constraints involving data collection or time

Example:
Passing rates on state licensing tests by teacher candidates are a proxy measure for the quality of teacher preparation delivered by teacher education institutions.

qualitative research - research in which the data are narrative descriptions or observations

Example:
A researcher observes how teachers instruct different reading curricula in two different schools. The researcher also interviews the teachers to understand their approaches to the different curricula and how approaches might be influenced by school characteristics.

quantitative data - data that are numbers and measurements

quantitative research - research in which the data are numbers and measurements

Example:
A researcher randomly assigns students to different reading curricula. At the end of the school year, the researcher examines the students' scores on a reading achievement test to determine whether the different curricula had different effects on reading.

quasi-experimental study - a research study in which (1) an independent variable is directly manipulated to measure its effects on a dependent variable and (2) participants are not randomly assigned to comparison groups

Example:
A researcher assigns 15 teacher preparation candidates who have senior seminar on Wednesdays to participate in eight weeks of student teaching. The researcher assigns 15 teacher preparation candidates who have senior seminar on Tuesdays to participate in 16 weeks of student teaching. After the candidates graduate, the researcher compares their scores on a performance-based teacher-licensing test. The amount of student teaching is the independent variable, and candidate performance on the teacher-licensing test is the dependent variable. The researcher does not randomly assign candidates to the comparison groups. As a result, differences between the groups on the test could be due to the amount of student teaching or due to other characteristics of the teacher candidates.

regression analysis - a statistical technique for determining the association between a dependent variable and one or more independent variables and thereby being able to predict variation in dependent variable by knowing the other variables

Example:
In School District X, a researcher collects data on beginning teachers' scores on the state licensing test (variable 1), number of college courses in mathematics (variable 2), amount of time spent in school-based field experiences prior to certification (variable 3) and the achievement gains in mathematics by each teacher's students (dependent variable). The researcher uses regression statistics to measure the association between the three teacher variables and student achievement gains and to estimate student achievement gains based on the contribution of each of the teacher variables to that association.

replicate - to repeat a research study using the same method and similar participants. A successful replication obtains the same results as the original study

research design - a plan of what data to gather, from whom, how and when to collect the data, and how to analyze the data obtained; for valid results, the design must be appropriate to answer the question or hypothesis being studied

sample size - the number of participants (e.g., students) or entities (e.g., schools) in a study sample; large samples are preferred because, if randomly selected, they are more representative of the population than small samples

selection bias - systematic effects on the dependent variable that occur due to characteristics of the study participants

Example:
A researcher conducts a study on the influence of student teaching on teaching performance. The researcher assigns 20 teacher preparation candidates who attend college during the day to participate in 16 weeks of student teaching. The researcher assigns 20 candidates who are night students to have eight weeks of student teaching. Selection bias in this study is likely because the characteristics of day and night students, such as age and motivation, may be different. The results could be due to these differences instead of the amount of student teaching.

simple descriptive study - a research study in which data are collected to describe persons, organizations, settings or phenomena

Example:
A researcher surveys administrators of 10 alternative teacher preparation programs to describe characteristics of the different programs.

standard deviation - a measure of the variability of the scores in a distribution (i.e., a set of scores) equivalent to the average distance of the scores from the mean

Example:
Scores: 9, 10, 10, 12, 14

For the example set of five scores, the mean is 11, and the standard deviation is 2. The scores vary on average about two points from the mean.

statistical model/analysis - the type of statistics used whereby data are analyzed to determine the relationship between variables in an experimental or quasi-experimental study; the appropriate statistics allow for generalizations about a population based on a sample drawn from that population

statistical significance - a result that has 5% or less probability of occurring by chance; because it is unlikely that a statistically significant result has occurred by chance, the result is said to reflect non-chance factors in the study, such as the effects of a treatment

structural equation modeling (SEM) - a statistical method generally used for confirmatory rather than exploratory purposes, to determine the extent to which data on a set of variables are consistent with hypotheses about the association among the variables

synthesis (research synthesis) - a comprehensive and systematic literature review of past empirical research studies on a specific topic; research syntheses can be quantitative or qualitative; meta-analysis is the term used for a quantitative synthesis, and narrative review is the term used for a qualitative synthesis

validity - the extent to which a study or measure accurately reflects or assesses the specific concept or variable the researcher is attempting to measure

variable - a characteristic or quantity that can change and have different values

Example:
Variables studied in education include characteristics of students (e.g., achievement), teachers (e.g., certification), schools (e.g., curriculum), districts (e.g., leadership), teacher preparation programs (e.g., accreditation) and states (e.g., education funding).

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