Measurement is fundamental to almost all forms of research and applied science. To conduct quantitative research, scientists must measure at least one variable. For example, researchers studying the effect of social rejection on self-esteem must measure participants' self-esteem in some way. Similarly, to apply scientific knowledge, practitioners often rely heavily on measurement. For example, school psychologists measure children's academic and cognitive aptitudes to place them in appropriate classes and to identify potential academic difficulties. Given the importance of measurement, researchers and practitioners must evaluate the quality of the measurement tools that they use. Reliability is a key facet of measurement quality, and split-half reliability is a method of estimating the reliability of a measurement instrument.