The research glossary defines terms used in conducting social science and policy research, for example those describing methods, measurements, statistical procedures, and other aspects of research; the child care glossary defines terms used to describe aspects of child care and early education practice and policy.
Instrument Error
A type of non-sampling error caused by the survey instrument (or questionnaire) itself, such as unclear wording, asking respondents for information they are unable to supply or the instrument being changed in some way during the course of the research.
Instrumental Variables
Instrumental variables are used to estimate causal relationships or effects when controlled experiments are not feasible or when a treatment is not successfully delivered to every unit in a randomized experiment's treatment group. An instrumental variable is related to the explanatory variable of interest (e.g., participation in a child care program or enrollment in a before- and after-school program) but has no independent effect on the dependent variable (e.g., children' reading and math scores or social skills). For example, region of the country may be associated with participation in child care or before- and after-school programs, but would not be expected to relate to measures of children's development.
Intent-to-Treat
Intent-to-treat (ITT) is an approach for estimating the effects of a treatment or intervention that includes all those who were randomly assigned to the treatment or control groups, regardless of the treatment they actually received. It includes those who were assigned to the treatment but who did not participate in the intervention and those who withdrew before the study was complete. Estimate of the treatment effect is generally conservative, because the outcomes of those who did not receive the treatment or the full treatment are generally smaller. It is best used when the researcher has access to outcome data on all those who were randomly assigned at the beginning of the study.
Inter-Rater Reliability
A measure of the consistency between the ratings or values assigned to a behavior that is being rated or observed; usually expressed as a percentage of agreement between two raters/observers, or as a coefficient of agreement which can be stated as a probability.
Interaction Effect
A situation where the effect of the independent variable on the dependent variable varies depending on the value of another, additional variable. For example, teaching style and student's gender would have an interactive effect if boys learned more in a lecture style classroom, while girls learned more in a discussion style classroom. In other words, the effect of teaching style on learning varies depending on student's gender.
Interaction Effect
An interaction effect is observed when the effect of one explanatory (independent) variable differs depending on the level of another explanatory variable. This is distinct from a "main effect," which is the effect of a single explanatory variable on the dependent variable. In the research literature, interaction effects are sometimes called simultaneous effects or moderators.
Intercept
The expected value of a dependent variable when all the independent variables are equal to zero.
Intercept
In regression analysis, the predicted value of Y (dependent variable) when all X variables (independent variables) are zero (0). It is also called the Y-intercept or the constant term. It is the place where the regression line crosses the Y-axis.
Internal Validity
The extent to which researchers provide compelling evidence that the causal (independent) variable causes changes in the outcome (dependent) variable. To do this, researchers must rule other potential explanations for the changes in the outcome variable.
Interval Scale
A scale of measurement where the distance between any two adjacent units of measurement is the same but the zero point is arbitrary. Scores on an interval scale can be added and subtracted but cannot be meaningfully multiplied or divided.