Before from previous prices. Malkiel points out

Before to explain the efficient market
hypothesis, there is an important to mention “efficiency”
in the financial markets such market do not allow investor to earn the average
returns above the average risks. There is an old story among economists, a $100
bill lying on the ground, the student stops to pick up then the professor said “Don’t
bother, if it were really $100 bill, it wouldn’t be there” the story shows
market is efficiency even valuation may sometimes exist errors. Market could be
efficiency even if many market participants are irrationality and stock prices
are volatility. Therefore, all of these concepts of market efficiency could be
summarized if market is efficiency that they do not allow investors to earn excess
risk adjust returns. Efficient Market Hypothesis has been accepted by
many financial economists in a generation ago, such as Fama (1970).  It reflects all information about stocks are
rationally and without delay, for example, in a security market, all investors
have rationality expectations, security prices can complete reflect all
available information, each security price is equal to the value of investment.
The efficient market hypothesis involved the theory of Random Walk, which
generally used in prices series, prices changing could represent prices goes randomly
from previous prices. Malkiel points out in the idea of random walk if
information is unimpeded and immediately reflect stock prices, the today’s prices
only reflect the news in today that cannot reflect news in tomorrow or future
because news is unpredictable, thus, resulting prices is unpredictable and
random. According Malkiel’s book (A
Random Walk Down Walk Street) published in 1973, he mentioned “a
blindfolded chimpanzee throwing darts could do as well as the experts select a portfolio”
the meaning of the advice is not really to throw darts that is to buy the basic
fund which involved all bought and held all stocks in the market that charged
low expenses.