Final answer:
Direct observation is used to quantify a child's performance by providing objective and measurable data on their actions during skill performance.
Step-by-step explanation:
To quantify a child's performance of a skill or a set of skills, the observation method typically used is direct observation. This method involves a direct, physical observation of the child's actions and behaviors while they are performing the skill. Unlike interviews or questionnaires, which gather subjective data through the responses of individuals, direct observation allows the observer to collect objective, measurable, or quantifiable data. This data can then be analyzed to assess the child's proficiency in the particular skill. Standardized tests are another tool used to quantitatively assess skills, but they are not an observational method as such and instead provide a structured way to measure performance against a standard set of criteria.