A further objection is often made concerning the quality of inspection made to produce the certificate. It cannot be invasive, so the inspector cannot drill walls or ceilings to determine the state or even existence of any insulation. The energy assessor can either assume the worst ('as built' to Building Regulations for the dwelling's age) or rely on the householder to produce documentary evidence on what may have been installed. This can produce uncertainty about the validity of the output from the assessor's analysis.