MARIO DRAGHI’S promise to avoid surprising investors as the European Central Bank (ECB) heads for the stimulus exit will require him to be clearer on an old-fashioned policy tool that hasn’t been touched in two years: interest rates.

The ECB president opened a conference in Frankfurt on Wednesday by saying policy adjustments will be “predictable, and they will proceed at a measured pace.” While the immediate task is to decide when to stop bond purchases, the real uncertainty lies in the institution’s vague expectation that borrowing costs will stay at current levels until “well past” the end of that program.

“A clear calendar guidance as to planning of the first hike and potentially the pace thereafter, which has already started but could still become a bit clearer, would be warranted,” Elga Bartsch, chief European economist at Morgan Stanley, said in a panel discussion at the event.

The reason for the focus on rates is that quantitative easing (QE) looks close to being maxed out. The ECB will buy debt until at least September at €30 billion ($37 billion) a month, and last week dropped a pledge to increase that pace if the economy deteriorates.

That signals the program is unlikely to grow much bigger than the currently intended €2.55 trillion.

Most economists predict purchases will end this year. The ECB’s internal staff calculations presented to the Governing Council last week assumed asset buying will total €30 billion in the fourth quarter, according to euro-area officials familiar with the matter.

QE remains critical in that the clock won’t start ticking on an interest-rate hike until fresh asset purchases — maturing debt will continue to be reinvested — come to a halt. At the moment, market pricing indicates that rates will rise about six months after that point, though that could change.

“Current ECB rhetoric is probably most consistent with a first rate increase in June 2019,” Greg Fuzesi, an economist at JPMorgan Chase & Co., said in a note. “But we see real scope for positive surprises to eventually pull this forward to March.”

The deposit rate has been at minus 0.4% since March 2016, and the main refinancing rate has been at zero.

Benoit Coeure, the ECB board member in charge of market operations, said last month that QE holdings are now so large that additional purchases can be slowed without boosting bond yields — but only if the guidance on interest rates is strengthened.

His colleague Peter Praet, the institution’s chief economist, reiterated at the Frankfurt conference that the language on rates will need to be further “specified and calibrated,” while warning that the process must be gradual.

“In the absence of perfect or complete knowledge about the way market participants will react to the announcement of monetary-policy changes, it may be optimal to adjust policy more cautiously and in smaller steps than if we had perfect knowledge,” he said.

Governing Council member Francois Villeroy de Galhau said on CNBC on Thursday that the ECB won’t pre-commit on when progress on inflation has been judged sufficient to end asset purchases.

Central-bank chiefs frequently say they have no desire to surprise investors, but do so anyway. Ben Bernanke sparked a “taper tantrum” of rising yields in 2013 when he first floated the idea of tapering the US Federal Reserve’s QE program. Draghi caused a mini-tantrum last year in Sintra, Portugal, when his reference to reflationary trends was interpreted as unexpectedly hawkish.

For some at the conference, QE has been a diversion as the ECB waited for its other measures to boost the euro-area economy and put inflation back on track toward its goal of just under 2%.

“QE was a big success, a publicity stunt, a placebo”’ said Daniel Gros, director at the Center for European Policy Studies. “You give the markets a sugar pill, they take it, they feel better, and that’s how a placebo works. It can at times be very effective, as we know, but usually the impact is transitory and not permanent.”

Draghi said the ECB has proven that its forward guidance is credible. That claim looks set to be tested as the economy grows and the institution edges toward the next policy change, though some of the responsibility will fall on his successor. Draghi’s eight-year term, which started back in 2011 with an immediate rate cut, ends in October 2019.

“The outlook for ECB policy over the next 18 months or so has never been so clear,” said Richard Barwell, an economist at BNP Paribas Asset Management. “And then once Draghi rides off into the sunset and there is a new sheriff in town all bets are off. It’s very hard to know how fast they exit or even how they exit.” — Bloomberg