One reason for the urgency behind Prime Minister Benjamin Netanyahu’s controversial speech to Congress on Tuesday is the fact that a nuclear deal between Iran and six world powers is reportedly taking shape in Switzerland. The parameters of the agreement under discussion — and, indeed, of any deal that can plausibly be reached right now —will leave Iran with infrastructure that could potentially be repurposed toward weaponization. So, the key metric by which the U.S. Congress will judge any agreement will be breakout time, the minimum period required for Iran, using that infrastructure, to produce 25 kilograms of weapons-grade uranium. That would be enough for a single bomb, should Tehran decide to build one.

The timescale for Iran to produce a bomb’s worth of fissile material is an appealingly simple criterion in light of the technical complexity of the negotiations. But it’s also a deceptively simple one. Five common misperceptions make breakout time a misleading gauge of the potential threat:

Misperception No. 1: Breakout time measures the time needed to build a nuclear weapon

Not really. Breakout time measures the time needed to produce fissile material for a bomb, not the bomb itself. After enriching enough weapons-grade uranium hexafluoride gas, Iran would have to turn the gas into powder form, convert the powder into a metallic core, assemble explosives around the core and finally integrate a miniaturized weapons package into the nose cone of a missile. Those steps would require a further 6 to 18 months after creating the fissile material, depending on how far Iran progressed in its alleged weaponization research that U.S. intelligence concluded had been shelved in 2003. Even if Iran got everything right on a first attempt, it would still need to test its bomb — as every nuclear-armed country has done — which would require more than one device and lengthen the time frame. (See Misperception No. 2.)

Misperception No. 2: Breakout time is measurable

Far from it; breakout time is estimated rather than calculated. Different experts using the same numbers come up with different time frames, even among the countries negotiating with Iran. They differ on assessments of average centrifuge efficiency and the time required to chemically covert uranium into feedstock, reconfigure centrifuge cascades and recycle waste. Breakout estimates, moreover, usually assume that an Iranian dash for the bomb would face none of the technical challenges that have plagued the program over the past decade.

More importantly, the breakout capacity measure ignores the reality that a single bomb does not make a nuclear deterrent. Assuming that Tehran would at minimum need two bombs’ worth of material in order to test one, the breakout time estimate doubles; assuming that Iran, like other nuclear-armed countries, would want a small arsenal, the time frame increases several times over.

Finally, while the number of centrifuges attracts disproportionate congressional attention, it is only one factor in the complex equation that determines breakout time. Other elements include the type and efficiency of centrifuges, the configuration of interconnected machines, the level of enrichment and the amount of stockpiled enriched material. And some of these elements are inversely correlated.

Misperception No. 3: Breaking out is Iran’s most likely path to weaponization

That’s a misguided assumption. The U.S. intelligence community has long concluded that Iran will not, in fact, use the intrusively monitored nuclear facilities under discussion in the current talks to pursue a nuclear weapon. Washington believes that should Iran decide to build nuclear weapons, it would be more likely to try to “sneak out” in a clandestine facility. Over the past three decades, this has been the route chosen by virtually every country dreaming of nuclear weapons, including North Korea, Iraq, Syria, Libya and Romania, with varying degrees of success. Both Iranian enrichment facilities, in Natanz and Fordow, were built covertly and declared only after being exposed by Western intelligence agencies. Fixation on a possible breakout distracts from the greater risk of a sneak-out and therefore from the two main safeguards for preventing one: transparency and monitoring.

Misperception No. 4: A shorter breakout time reduces Washington’s ability to prevent an Iranian sprint to nuclear weapons

Incorrect. Under all conceivable agreements — and even under the status quo inspection regime — the discovery of any trace of uranium enriched beyond civilian-grade would trigger an alarm. Skeptics counter that such evidence would likely be ambiguous and require time-consuming analysis, which combined with the West’s aversion to confrontation may prevent the mobilization of forceful action in time. But the scale of enrichment activity required to produce bomb material would require a brazen breakout, significantly increasing the prospects for speedy detection under a watchful monitoring regime. Such evidence would prompt and in the eyes of world powers legitimize a firm response, for which the U.S. and Israel probably already have extensive contingency plans. Given the extensive long-term U.S. deployments in the Persian Gulf, air strikes could be launched — with or without international blessing — in a matter of days. And that capacity to strike means there’s little practical difference between six, 12 or 24 months of breakout time.

Misperception No. 5: If the breakout time is short enough, Iran will dash to build a bomb

This idea is not supported by the factual track record. Iran’s nominal breakout time over the past four years has, in fact, been less than six months but that did not prompt weaponization. Now it’s negotiating a deal that would extend the breakout time, by the same measures, to one year and subjects it to enhanced monitoring. Since 2007, the U.S. intelligence community has consistently assessed Iran to have the scientific, technical and industrial capacity to produce nuclear weapons, if it chooses to do so, but that no such political decision had been taken. In addition, U.S. National Intelligence Estimates have concluded that Iranian leaders’ decisions on whether to build nuclear weapons will be based on their perception of their threat environment and on a cost-benefit analysis. If leaders in Tehran believe that their survival requires the ultimate deterrent, they would likely be willing to endure even more punishing sanctions to acquire the bomb.

Beyond technical parameters of a nuclear deal, the question facing Western powers is how to shape Tehran’s perception of its threat environment. By that logic, pursuing a more expansive engagement with Iran on economic, political and security questions may become even more important than lengthening breakout time.

Ali Vaez is the International Crisis Group’s senior Iran analyst. He tweets at @AliVaez.