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Stock Market Forecastability and Volatility: A Statistical Appraisal

TLDR
In this article, the authors present and implement statistical tests of stock market forecastability and volatility that are immune from the severe statistical problems of earlier tests, and show that although the null hypothesis of market efficiency is rejected, the rejections are only marginal.
Abstract
This paper presents and implements statistical tests of stock market forecastability and volatility that are immune from the severe statistical problems of earlier tests. It finds that although the null hypothesis of market efficiency is rejected, the rejections are only marginal. The paper also shows how volatility tests and recent regression tests are closely related, and demonstrates that when finite sample biases are taken into account, regression tests also fail to provide strong evidence of violations of the conventional valuation model.

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NBER
Working
Paper
#3154
October
1989
STOCK
MARKET
FORECASTABILITY
AND
VOLATILITY:
A STATISTICAL
APPRAISAL
ABSTRACT
This
paper
presents
and
implements
statistical
tests
of
stock
market
forecastability
and
volatility
that
are
immune
from
the
severe
statistical
problems
of
earlier
tests.
Although
the
null
hypothesis
of
strict
market
efficiency
is
rejected,
the
evidence
against
the
hypothesis
is
not
overwhelming.
That
is,
the
data
do
not
provide
evidence
of
gross
violations
of
the
conventional
valuation
model.
N.
Gregory
Mankiw
Department
of
Economics
Harvard
University
Cambridge,
MA
02138
(617)495-4301
David
Romer
Department
of
Economics
University
of
California
Berkeley,
CA
94720
(415)642-1785
Matthew
D.
Shapiro
Department
of
Economics
University
of
Michigan
Ann
Arbor,
MI 48109
(313)764-5419




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References
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