Iasonas,
In response to PS.1
try error.bars in the psych package.
Bill
At 1:04 AM -0700 3/30/10, Iasonas Lamprianou wrote:
Dear friends,
I have a statistical question. Sometimes, if I compare boys to girls
on a specific variable, the error bars (confidence interval of
means) seem to overlap slightly. Still, when I run a t-test, I find
statistically significant differences. The rule is clear: if the
confidence intervals do not overlap, then there is statistically
significant difference. But if they overlap slightly, we have to use
a t-test to know for sure if the the two means differ significantly.
The point is: is there a rule of thumb to say, for example, "if the
overlap is less than 20% of the length of the standard error, then a
t-test would give significant results?"
thank you for your time
P.S.1 is there an easy way to plot error bars in R?
P.S.2 an interesting discussion about this - highly recommended to
read it - can be found at
http://scienceblogs.com/cognitivedaily/2007/03/ill_bet_you_dont_understand_er.php
jason
Dr. Iasonas Lamprianou
Assistant Professor (Educational Research and Evaluation)
Department of Education Sciences
European University-Cyprus
P.O. Box 22006
1516 Nicosia
Cyprus
Tel.: +357-22-713178
Fax: +357-22-590539
Honorary Research Fellow
Department of Education
The University of Manchester
Oxford Road, Manchester M13 9PL, UK
Tel. 0044 161 275 3485
iasonas.lampria...@manchester.ac.uk
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