This case is an appeal by the state because the trial court excluded evidence of an Intoxilyzer test result in a prosecution for driving while under the influence after the trial court ruled that the pretest observation period, as required by OAR 257-030-0070(2)(a),1 was not met. We reverse.
At the hearing on the motion to exclude evidence of the Intoxilyzer test result, Officer Bigman, the operator of the test, and Officer Lenhardt, who observed the administration of the test, testified. Bigman said that he inspected defendant’s mouth at 10:20 p.m. and that it showed no indication that defendant had vomited, regurgitated or was ingesting anything at that time. Bigman remained in the room with defendant until 10:55 p.m. when he left for “approximately two to three minutes” to take a telephone call. Before he left, he instructed Lenhardt to “watch the defendant.” Lenhardt testified that he was familiar with the pretest observation requirement embodied by the rule and understood the procedure that was occurring. Bigman testified that when he returned to the room, Lenhardt “did not tell me anything.” He said that he understood Lenhardt’s silence on his return to mean “that [defendant] did not move, he did not vomit, puke, did not do anything. Officer Lenhardt would have told me if he even coughed.” Bigman rechecked defendant’s mouth at 11:02 p.m., and again, there was no indication of anything that would implicate the rifle. “Shortly thereafter,” he administered the Intoxilyzer test. Lenhardt, who had been with defendant when the observation period began at 10:20 p.m. and had stayed with him during the entire time, testified that defendant did not regurgitate, vomit or ingest anything during the observation period. The *696officers’ testimony is uncontroverted. Defendant did not testify, and he presented no evidence that any act on his part occurred that would have implicated section 2 (a) of the rule or affected the test result.
The trial court ruled:
“The pre-test observation requirement under OAR 257-30-070 was not met in this case in that the operator of the [Ilntoxilyzer (the arresting officer) could not have been ‘certain’ that the prohibited activities (e.g.-regurgitation) did not take place during the 15 minutes before administration of the test.”
It explained that, “The two officers did not communicate with each other about what to watch for while the arresting officer was out of the room, nor did they discuss what had occurred when the arresting officer returned to the room.”
Our standard of review regarding a trial court’s ruling on whether the state complies with the pretest requirements for an Intoxilyzer test focuses on a question of fact and a question of law that afford meaning to the rule. See State v. Balderson, 138 Or App 531, 534, 910 P2d 1138, rev allowed 324 Or 305 (1996) (interpreting the “operator must make certain” language of former OAR 257-30-020(l)(b) which also required a 15-minute pretest observation period).2 First, the rule requires that the operator of the test form a subjective belief to the degree of “certainty” that the test subject has not engaged in any of the acts described by the rule. Second, that belief must be reasonable under the circumstances. Id. at 536.
*697Also, the rule expressly contemplates that the operator may rely on the observation of someone else in reaching that required subjective belief. OAR 257-030-0070(2)(b); see also State v. McVay, 83 Or App 312, 315, 731 P2d 466 (1987) (interpreting former OAR 257-030-020(1)(b), that required that “[t]he test operator must make certain”); see also State v. Herring, 112 Or App 83, 85-86, 827 P2d 932 (1992) (holding that the former rule had been complied with when the test operator asked the observer whether the defendant “had done anything?”). However, the text of the rule does not require express communications between the observer and the test operator about what occurred during the absence of the operator, and it is not difficult to think of situations where silence is as effective a communication as oral statements, particularly in situations where procedures and roles are commonly understood by those involved in the administration of the test.3 Thus, whether “certainty” is reasonable under the circumstances must be tested by all the circumstances of which the operator was aware at the time that the operator formed the belief that the requirements of the rule had been met.
In this case, the trial court ruling does not express a finding about the subjective component of the rule.4 It did not hold that the operator “was not certain” that the requirements had been met. Rather, it held that the operator “could not have been certain” because the officers did not expressly communicate with each other about the requirements of the rule. That seems to us to be a legal conclusion, i.e., that the rule requires an oral communication between the test operator and the observer and that the operator could not be “certain” that the requirements of the rule had been met in the *698absence of such a communication. Such an interpretation would write a requirement into the rule that is not expressed by the text of the rule and suggests that the trial court did not correctly understand the import of the rule.
Accordingly, we remand to the trial court. In the event that the trial court finds that Bigman held the requisite subjective belief, it must then determine whether his belief was reasonable under the circumstances. Although the facts that Bigman did not tell Lenhardt what to watch for while Bigman was out of the room, and that Lenhardt did not expressly tell Bigman that nothing of significance had occurred in his absence, comment on the reasonableness of Bigman’s belief, they are not necessarily dispositive. An inquiry about “reasonableness” requires consideration of the totality of the circumstances that were presented to Bigman, including the evidence that Bigman told Lenhardt to “watch the defendant” during his absence, that Lenhardt testified that he was “familiar with the pre-[I]ntoxilyzer test requirement of the observation period,” and that Bigman understood Lenhardt’s silence to mean “that the defendant * * * hadn’t done anything “ and “Officer Lenhardt would have told me if he had even coughed.”
Additionally, evidence offered by the state can arise to the level of a prima facie showing that compliance occurred. In that event, it is incumbent on a defendant to overcome that showing by offering evidence that the rule’s requirements were not met. State v. Demings, 116 Or App 394, 397, 841 P2d 660 (1992), rev den 315 Or 443 (1993). We cannot determine from the trial court’s order whether the trial court considered that rule of law. On remand, the trial com! must determine, by applying the above rules, whether Bigman held a subjective belief of certainty about whether defendant had taken anything by mouth, vomited or regurgitated during the observation period and whether that belief was reasonable.
The state also urges that Oregon Laws 1997, chapter 313, section 1, requires the admission of the evidence.5 We do *699not reach that issue because it is not necessary to the resolution of the issue before us.
Reversed and remanded.
OAR 257-030-0070(2)(a) provides, in part:
“(2) Pre-test Requirement:
“(a) The operator is certain that the subject has not taken anything by mouth (drinking, smoking, eating, taking medication, etc.), vomited, or regurgitated liquid from the stomach into mouth, for at least fifteen minutes before taking the test;
“(b) There is no requirement that the operator be the person who makes observation of the subject.”
In State v. Balderson, 138 Or App 531, 534, 910 P2d 1138, rev allowed 324 Or 305 (1996), the defendant testified that she had inaudibly regurgitated during the observation period. Despite that testimony, the trial court denied the defendant’s motion to suppress the results of the test because the test operator had made an objectively reasonable effort to “make certain” that she had not regurgitated. We affirmed the trial court, holding that the rule focused not on whether in fact the subject took anything by mouth, vomited or regurgitated, but “on the conduct of the officer.” (Emphasis in original.) In that case, the issue was whether the rule required merely that the officer be “certain” that none of the enumerated events had occurred or whether the rule required that in fact, the events had not occurred. Similarly, the issue in this case focuses on the state of mind or belief that Bigman held.
Contrary to the assertion of the dissent, we did not hold in State v. McVay, 83 Or App 312, 315, 731 P2d 466 (1987), that the rule contemplates only express oral communications. In response to the defendant’s contention in that case that there was an insufficient factual basis for the operator to be certain, we noted that the test operator failed to inquire whether the defendant had vomited or regurgitated. Under those circumstances, the operator could not have been certain that all the requirements of the pretest observation period had been met. We said, “At a minimum, that requires that that information be communicated to him.” Id. at 316. Our holding said nothing about the form in which the communication must occur.
Our opinion in Balderson issued on January 24, 1996. In this case, the trial court heard argument on October 4,1996, and rendered its formal order on October 29,1996.
After the trial court ruled, Oregon Laws 1997, chapter 313, became effective on June 12,1997.