For example - as the illustration shows - the end of summer tends to signal a fall-off in convertible values. The reliability of this factor is remarkable for its consistency.
But, as they say in the small print of financial investment products, past performance is no guarantee of future results.
This is because the burgeoning convertible market is seeing a lot of new entrants that are significantly different from the traditional offering in one crucial respect.
Take Ford Focus CC, Vauxhall Astra Twintop, Volkswagen Eos and the already established Peugeot 307 and Renault Mégane CC. The feature they all share is the metal folding top.
Research shows that convertible seasonality, unsurprisingly, relates directly to the weather. It is too early to do more than pose the question, but it may be possible that metal folding tops will flatten out the value fluctuations for convertibles.
Of course, other factors may come into play. For example, if these mechanically complex pieces of equipment suffer long-term reliability issues, then the presumed advantage of sealing the elements out more securely will be dissipated.
The outcome remains to be seen but we are keeping a watching brief on this particular issue.
Four-seat convertible seasonality
The graph shows that there is usually a severe dip in convertible values once summer is over