Apparently forecasting iPhone sales is harder than it looks.

From the health of the consumer to the average time it takes a current user to upgrade, analysts covering the stock have much to consider when making their estimates.

And yes, that includes Brexit.

"We are lowering our estimates for June and September quarters given potential for lower demand from macro uncertainty Brexit related , currency volatility and lengthening replacement cycles," Citigroup Inc. analyst Jim Suva said in a note.

He also points out that the average iPhone replacement rate has risen from roughly 24 months in 2013 to 28 months now.

Furthermore, his models suggest that range could increase to as much as three years—not good news for Apple.

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