Recent empirical studies show that the squares of high-frequency stock retu
rns are long-range dependent and can be modeled as fractionally integrated
processes, using, for example, long-memory stochastic volatility models. Ar
e such long-range dependencies common among stocks? Are they caused by the
same sources of variation? In this article, we classify daily stock returns
of Standard and Poor 500 companies on the basis of a company's size and it
s business or industrial sector and estimate the strength of long-range dep
endence in the stock volatilities using two different methods. Almost all o
f the companies analyzed exhibit strong persistence in volatility. We then
use a canonical correlation method to identify common long-range dependent
components in groups of companies, finding strong evidence in support of co
mmon persistence in volatility Finally, we use a chi-squared test to study
the effects of company size and sector on the number of common long-range d
ependent volatility components detected in groups of companies. Our results
indicate the existence of some size effects, although they are not related
to company size in a monotonic manner. On the other hand, the effects of c
ompany sector are pronounced. Randomly selected companies are found to be d
riven by a significantly larger number of persistent components than compan
ies in certain business sectors, implying that persistence in stock volatil
ity of companies in the same sector is more likely caused by the same sourc
e. These results suggest, among other interesting implications, that the vo
latilities of stocks for companies in the same business sector will be more
often tied together in the longer run than will the volatilities of compan
ies grouped only on the basis of size.