Observational measures in HDFS include participant observation, naturalistic observation, and structured observation. The Strange Situation, the visual cliff, and violation of expectations are iconic structured observational methods in the study of child development. The major risk of observation is reactivity, which can be reduced through acclimation or by the use of unobtrusive instruments. Another risk is observer bias, which can be reduced through the use of explicit coding standards, multiple coders, and blinding. The primary advantages of observational measurement are the improved objectivity, and the ability to measure variables that are impossible to measure with self-report.