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.
Pooled Cross-Sectional Data
Pooled cross-sectional data (repeated cross-sectional data) are collected from different (independent) samples from the same population at different points in time. These data are contrasted with panel data that are collected from the sample at different time points. The early care and education program participation surveys administered as part of the National Household Surveys Program are examples of pooled cross-sectional data.
Population
In statistics, the population includes all members of a clearly defined group. The population can be comprised of a group of individuals (e.g., all children ages zero to 5) or of organizations (e.g., all programs providing early childhood education to 3- and 4-year old children). Samples are drawn from the population and the statistical results that are derived from random samples can be used to estimate characteristics of the whole population.
Potential Outcomes Framework of Causality
The potential outcomes framework of causality is based on the idea that every subject has different potential outcomes depending on their "assignment" to a condition (for example, assignment to treatment and control group or to different types or levels of treatment). It recognizes that the effect of assignment on an individual subject can only be directly observed for one condition. Statisticians use this framework to assess causality in observation (nonexperimental) studies.
Power
The degree to which a statistical test will detect significant differences between groups in a sample, when the differences do in fact exist. Sometimes statistical tests are not "powerful" enough to detect significant differences between groups in a sample that actually do exist in the population. The primary reason that a statistical test is not powerful is a small sample.
Practical Action Research
Practical action research is intended to address a specific problem. For example, a team of researchers, teachers, and parents may work together to design and execute a study whose purpose is to improve the communication between children's classroom teachers and their parents with an emphasis on ongoing bi-directional communication.
Precision
Precision of measurement is the degree to which repeated measurements under the same set of conditions show the same results. The standard error of measurement (SEM) is one way of calculating the precision of a measure or instrument (e.g., assessment of children's early literacy skills). The smaller the SEM, the more precise the measurement of the instrument. Precision of estimates refers to how close estimates derived from different samples are to each other. The standard error is a measure of precision of sample estimates. When the standard error is small, sample estimates are more precise; when the standard error is large, sample estimates are less precise.
Predictive Validity
To find out if a test or other measure assesses what it is intended to assess the correlation between the test score or value of the measure and a future external criterion. For example, the predictive validity of an assessment administered to children during the fall of the kindergarten year might be evaluated by correlating children's scores on this assessment with their scores on a standardized test administered in third grade. In general, the higher the correlation the stronger the predictive validity of the test.
Predictor Variable
The variable whose effect on an outcome variable is being modeled. A predictor variable is also called an "independent" variable.
Pretest
In research, pretest has several meanings. In experimental or evaluation research, it refers to a measure (e.g., children's performance on a test of their early literacy skills) taken before the experimental manipulation or intervention is applied or is introduced. It is also used to refer to the process of testing materials (e.g., questionnaires) and procedures (e.g., approach for collecting information about children from program records for use in selecting a sample of children) before they are used in a main study. For example, a parent questionnaire that will be used in a large study of preschool-age children may be administered first to a small (often non-random) sample of parents in order to identify any problems with how questions are asked and whether the response options that are offered to parents are adequate.
Primary Sampling Units
Primary sampling units refer to the sampling units that are selected at the first (primary) stage of a multi-stage sample. For example, a study of the instructional practices in early childhood classrooms might use a multi-stage sample design to select the classrooms to be studied. Using this approach, early childhood programs are selected first and then classrooms are selected from all the classrooms in the selected programs. Programs are the primary sample units (PSU) and classrooms are the secondary sampling units.