Assessing the mean and a measure of
error using a plot is a good initial look at the data, but is not sufficient to
determine whether the difference is statistically significant. The choice
of the error bar is important in this visual estimation, and the appropriate
error term to plot is the standard error of the mean, not the standard
deviation. The SD provides the variance in the sample, whereas the
standard error of the mean provides an estimate of the true mean of the
population from which the sample was taken. The statistical analysis makes
an inference about the population based on the data from the sample; therefore,
population estimates are the better method to assess differences.
Frank Underwood
From: Evidence based
health (EBH) [mailto:[log in to unmask]] On Behalf Of Markos
Kashiouris
Sent: Tuesday, March 31, 2009 10:31 AM
To: [log in to unmask]
Subject: Question
Frequently in basic research papers, data are graphically
presented in a bar-format with one standard-deviation up and down represents
the error bar. I have been working in a lab in the past. The data were
presented in such way that if the SD error bars didn't overlap in the
comparison groups eg cell counting etc the researchers presented their data as
significant.
These basic-science research papers are the basis of the
pyramid which leads to animal studies and higher phase trials including human
research. I was wandering if this reporting method is statistically justified
and if not what the error bars shall include
: 2 standard deviations each side (up or down from the bar),
one SD each side of the bar or using the Standard Error?
Thank you.
Markos Kashiouris, MD
PGY2 Resident in Internal Medicine