1.4 Why this chapter?
We believe that there can be one scale for measuring child development and that this scale is useful for many applications. We also believe that there cannot be one instrument for measuring child development that is suitable for all situations. In general, the tool needs tailoring to the setting.
We see that practitioners often view instruments and scales as exchangeable. In daily practice, the practitioner would pick a particular tool to measure a specific faculty, which then effectively produces a “scale score.” Each tool produces its own score, which then feeds into the diagnostic and monitoring process.
We have always found it difficult to explain that scales and instruments are different things. For us, a scale is a continuous concept, like “distance,” “temperature” or “child development,” and the instrument is the way to assign values to the particular object being measured. For measuring distance, we use devices like rods, tapes, sonar, radar, geo-location, or red-shift detection, and we can express the results as the location under the underlying scale (e.g., number of meters). It would undoubtedly be an advance if we could establish a unit of child development, and express the measurement as the number of units. If we succeed, we can compare child development scores, that are measured through different devices. This chapter explores the theory and practice for making that happen.