Despite all the investing information that's
available — or maybe because of it — it's unclear
how many investors really understand the constant fluctuations
of the securities markets, and what they can reasonably expect
to earn on their investments.
It's
true that historical trends show that over time securities, stocks
in particular, tend to go up in value. But the shorter the time
horizon, the more difficult it is to predict — even for investment
professionals — what direction the market may be headed.
Only one thing is certain: The values of securities will go up
in some years, just as surely as they will go down in others.
What can help ensure that your expectations
are in line with reality is understanding the history of investment
market performance and how to use market benchmarks as a measure of current performance.
Jeremy Siegel, The Wharton School
The term
benchmark comes from
a surveyor's mark indicating height above sea
level. For investors, however, a benchmark is an index, average,
or other measure that's used as a standard
for evaluating the performance of the securities markets.
For example, the Dow Jones Industrial Average is a
widely followed benchmark for large-cap U.S. stocks.
It's
true that historical trends show that over time securities, stocks
in particular, tend to go up in value. But the shorter the time
horizon, the more difficult it is to predict — even for investment
professionals — what direction the market may be headed.
Only one thing is certain: The values of securities will go up
in some years, just as surely as they will go down in others. 
Jeremy Siegel, The Wharton School 