Hawke's Bay, a region of highly variable and sporadic rainfall, teetered on the edge of a drought in January 2017 when at its driest on record.

On average, there were 5.4 millimetres of rain in Napier – the lowest amount recorded since records were kept, according to MetService.

While rainfall on a national basis was much higher than normal, a MetService meteorology described 2017 as "the year it didn't stop raining".

But the story is more complicated than that. In many normally fertile farmland areas like Hawke's Bay, rain volume last year was well below average.

Like farmers planning a harvest, investors pinning their expectations on statements about arithmetical "average" investment returns can be disappointed.

As with rainfall, market returns are rarely evenly distributed either across time or place.

Of course, one could take one's cues from the financial news, which tellingly is normally scheduled just before the weather forecast on the nightly TV news bulletin.

But there isn't much evidence of anyone being able to consistently and reliably forecast market returns one year to the next, never mind one day to the next.

For instance, going all the way back to 1926, the US sharemarket index, the S&P 500, has had an annualised compound return of 10.2 per cent in US dollar terms.

Yet, on only six occasions in the intervening nine decades has the individual calendar year return been within two percentage points of that result. As a single number, an average result ignores the distribution of possible outcomes. For example, the S&P 500 has been up or down by more than 20 per cent in a calendar year on 40 occasions in this nine-decade period.

In Australia, from 1980 until the end of 2017, the benchmark S&P/ASX 300 index has had an annualised return of 11.3 per cent in Australian dollar terms. Yet only in five years of that near four-decade period have individual calendar year returns been within two percentage points of that average.

Individual year performances range from as low as -38.9 per cent during the year of the Global Financial Crisis in 2008 to +66.8 per cent in 1983!

Just as precipitation varies across place, so too do market returns.

Guess what happened in 2016? Canada vaulted from worst to best performing market, with a return of 25.2 per cent. But Denmark slid all the way from top to bottom, with a return of -15.4 per cent.

Overall, developed markets gained just over 8 per cent in 2016.

It's hard to pick, isn't it? Perhaps there's a lesson from farmers here.

Faced with climate variability and unpredictable patterns in precipitation, some of them diversify from single-crop agricultural systems to several different crops, or they rotate from crops to grazing livestock.

Others build completely new businesses on their land, such as poultry farming, tourism or renewable energy.

Likewise, for investors, an expectation of "average" returns every year is likely to lead to disappointment. But if they accept that they do not know when or where the "rains" will fall, they can focus instead on building structured, diversified strategies.

This way, they are more likely to capture the returns wherever they happen to occur in markets around the world.

• The information provided, or any opinions expressed in this article, are of a general nature only and should not be construed or relied on as a recommendation to invest in a financial product or class of financial products. You should seek financial advice specific to your circumstances from an Authorised Financial Adviser before making any financial decisions. A disclosure statement can be obtained free of charge by calling 0800 878 961 or visit our website, www.stewartgroup.co.nz