Layer of Protection Analysis (LOPA) is undoubtedly one of the most successful techniques for assessing risk, nevertheless it is an estimation tools therefore it is susceptible to "garbage in, garbage out" (GIGO) phenomenon.
George Fuechsel, an early IBM programmer and instructor is said to have used GIGO as a concise way of reminding his students that a computer just processes what it is given in other words, a faulty decision can be made as a result of incomplete information
In case of LOPA analysis, assigning inappropriate probability of failure on demand (PFD) values would result in useless results. Wrong PFD numbers or improper consideration of safeguards may not provide the desired level of protection or it may provide excessive protection that is not economically justifiable.
Layer of protection analysis (LOPA), if not properly applied could become a mere number crunching (playing with PFD values) exercise. Having experienced facilitators, an experienced LOPA team and updated relevant documents helps ensure LOPA will be done properly. Keep in mind that the PFD numbers are average values. For a number of safeguards, average PFD values are available in literature and tend to be conservative.
Finally, the proper selection of instruments and proper application, installation and maintenance are the key elements, which must be used in combination with LOPA to enhance safety.
LOPA is a tool that helps you shape your perception about the level of safety you are achieving.