Therefore the investigator must estimate whether the type of observation selected will significantly alter the task under study. Observational situations which produce rich context data are rarely the ones which produce precise and controlled data. So data will be incomplete, inconsistent and sometimes unusable Observational techniques cannot provide information about underlying thought-processes, and so they will be of very little use for highly cognitive activities. The equipment needed for producing high quality observational data can be expensive and difficult to install, and video recording followed by later analysis is time-consuming. At the simplest level passive observation, where the person conducting the observation quietly observes and records the user activity, can be used to record some details of the processes which the user undertakes.
Passive observation has the advantage that it minimises the intrusion which the user experiences.
One problem with passive observation is the practical difficulty in recording sufficient detail of the activity. This can be overcome through the video recording of user activity which, after transcription into textual form, can provide a highly detailed analysis. The investigator takes part in the tasks alongside other operators. This type of observation can also be useful if aspects of team performance are being investigated to understand how the team members are organised and perform their tasks. There is also a difficulty in labelling an interaction and a message especially if they do not fit into the request or promise categories.