When trying to predict the future, most investors and Wall Street analysts like hard data. A forecast based on recent sales trends seems far more "scientific" than predictions based on rumored features of future products and guesses about what consumers really want.

However, extrapolating from sales trends has been a lousy strategy for predicting Apple's (NASDAQ:AAPL) results. Indeed, sales of Apple's most important product, the iPhone, have been extremely volatile in recent years.

The iPhone's past does not predict its future

If there is one lesson that investors could learn from looking at iPhone sales trends over the past five years, it's that they don't follow any regular pattern.

Fiscal Year

iPhone Unit Sales Growth

iPhone Revenue Growth

2012

73%

71%

2013

20%

16%

2014

13%

12%

2015

37%

52%

2016

(8%)

(12%)

Data source: Apple annual 10-K filings.

Five years ago, the iPhone was still a relatively new product. At that time, the rising popularity of smartphones and the steady addition of new iPhone wireless carrier partners was driving explosive growth. For Apple's 2012 fiscal year, unit sales and revenue for the iPhone product line both surged more than 70%.

iPhone sales growth cooled dramatically in the following two years. In fiscal 2014, iPhone unit sales rose just 13%, while revenue increased 12%.

In a 2014 article for Quartz, Christopher Mims noted that "Apple has stopped growing." Mims claimed that new, larger-screened phones that were then rumored to be on the way "mean there's still potential for incremental growth in Apple's revenue. But that's it."

He couldn't have been more wrong. In the following year -- Apple's fiscal 2015 -- iPhone unit sales growth accelerated to 37%. And thanks to higher prices for the new iPhone 6 Plus (and rising customer demand for higher-memory configurations), iPhone revenue surged 52%.

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The iPhone 6 drove incredible sales growth for Apple two years ago. Image source: Apple.

In light of this strong momentum, most analysts expected further iPhone sales growth (albeit more subdued growth) in fiscal 2016. They were wrong again. Last year, iPhone unit sales slipped 8% year over year and revenue fell by double digits.

Are analysts making the same mistake again?

Wall Street analysts don't expect iPhone sales to continue sliding, but they aren't expecting much growth going forward, either. Not surprisingly, there is a wide range of estimates, but on average, Apple analysts expect mid-single-digit iPhone sales growth over the next two years.

After overestimating Apple's potential iPhone sales for fiscal 2016, analysts may have swung too far in the other direction now. As Apple's performance a few years ago showed, last year's sharp slowdown in iPhone sales could still be followed by a return to strong growth in the future.

Reasons for optimism about Apple's next-gen iPhone

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Most reviewers think that the iPhone 7 doesn't have any must-have features. Image source: Apple.

Most reviewers agree that the iPhone 7 isn't a big upgrade over iPhone 6-series phones. There are new colors, Apple has bumped up the base storage to 32 GB, and the headphone jack is gone, but the exterior design and key components like the screen haven't changed much.

Despite this relatively modest upgrade, iPhone sales seem to be on track to return to growth in the current 2017 fiscal year. The explosive growth of the iPhone user base over the last few years means that there are more iPhone owners looking to upgrade than ever before. This seems to have offset the lack of a "wow" factor for the iPhone 7 and iPhone 7 Plus.

Looking ahead, many industry observers expect Apple to release a "revolutionary" new iPhone design next year, to coincide with the iPhone's 10th anniversary. New features may include an all-glass casing with no bezels, wireless charging, and curved OLED screens.

If these rumors prove to be accurate, iPhone sales could accelerate again as they did two years ago, due to upgrade demand shifting into fiscal 2018 from the adjacent years, along with an uptick in "Android-switching." Will iPhone sales grow 37% year over year again? Probably not, but a year-over-year growth rate of around 20% is entirely plausible.

While a few analysts on Wall Street recognize this possibility, the vast majority have been scared away from making bold iPhone sales growth projections after Apple's disappointing fiscal 2016. However, the volatility of Apple's iPhone sales in the past few years shows that sales trends over the course of a year -- or even two -- offer very little guidance about what to expect in the future. Investors shouldn't ignore the past, but they shouldn't read too much into recent iPhone sales trends, either.

Adam Levine-Weinberg is long January 2017 $85 calls on Apple and short January 2017 $110 calls on Apple. The Motley Fool owns shares of and recommends Apple. The Motley Fool is long January 2018 $90 calls on Apple and short January 2018 $95 calls on Apple. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.