NFPA 921-2014 Section 4.5 et seq. applies to all opinions.

NFPA 921-2014
Chapter 4 Basic Methodology
4.5 Level of Certainty. The level of certainty describes how
strongly someone holds an opinion (conclusion). Someone
may hold any opinion to a higher or lower level of certainty.
That level is determined by assessing the investigator’s confidence
in the data, in the analysis of that data, and testing of
hypotheses formed. That level of certainty may determine the
practical application of the opinion, especially in legal proceedings.

4.5.1 The investigator should know the level of certainty that
is required for providing expert opinions. Two levels of certainty
commonly used are probable and possible:

(1) Probable. This level of certainty corresponds to being
more likely true than not. At this level of certainty, the
likelihood of the hypothesis being true is greater than
50 percent.

(2) Possible. At this level of certainty, the hypothesis can be
demonstrated to be feasible but cannot be declared probable.
If two or more hypotheses are equally likely, then the
level of certainty must be “possible.”

4.5.2 If the level of certainty of an opinion is merely “suspected,”
the opinion does not qualify as an expert opinion. If
the level of certainty is only “possible,” the opinion should be
specifically expressed as “possible.” Only when the level of certainty
is considered “probable” should an opinion be expressed
with reasonable certainty.

Pat Kennedy, CFEI, CFPS, MIFireE
Fire and Explosion Analyst
Sarasota, Florida
[www.kennedy-fire.com]