Former Reserve Bank economist Michael Reddell says it's getting harder to know how much the economy's growing or shrinking.

Stats NZ estimates GDP rose 0.2 percent in the December quarter, below most forecasts.

But Reddell says it's also revised the figures for previous quarters.

He says the 0.2 for December could still potentially be revised up as high as 0.5 percent, or as low as minus 0.1.

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