Stats: Confidence
interval for a rate (October 10, 2007). Dear Professor Mean, How do
you calculate a confidence interval for a rate?
Stats: Is my confidence
interval wide? (September 11, 2007). Dear Professor Mean, I have a
case-control design. Among the cases, 271 were exposed and 317 were
unexposed. Among the controls, 125 were exposed and 976 were unexposed. After
adjustments for covariates, this produced an odds ratio of 7.4 with a 95%
confidence interval of 5.7 to 9.5. Is this a wide interval?
Stats: Is a 10% shortfall
in sample size critical? (October 23, 2006). Dear Professor Mean, I'm
reviewing a paper where they did a power calculation based on 60 patients per
group, but in the research study, they ended up only getting 55/58 per group.
Since their sample size was much less than what they originally planned for,
does this mean that the study had inadequate power?
Stats: Is my confidence
interval too wide? (September 21, 2006). Dear Professor Mean, Is there
a rule of the thumb to judge if a 95% CI is wide or narrow?
Stats: An exact confidence
interval for a binomial proportion (August 18, 2006). A researcher came
into my office this morning with some data that was strongly negative. Out of
15 patients, none showed a detectable improvement after the use of a
controversial treatment. That sounds like a strong negative result to me, but
a reviewer asked a legitimate question: How do you know that you are not
having problems with a Type II error?
Stats: Confidence
interval for a correlation coefficient (July 11, 2005). In many
exploratory research studies, the goal is to examine associations among
multiple demographic variables and some outcome variables. How can you
justify the sample size for such an exploratory study? There are several
approaches, but one simple way that I often use is to show that any
correlation coefficients estimated by this research study will have
reasonable precision. It may not be the most rigorous way to select a sample
size, but it is convenient and easy to understand.
Stats: Examples of
confidence intervals (June 28, 2005). The following abstracts, all from
open source journals, provide good teaching examples of how confidence
intervals are used in research publications.
Stats: Confidence intervals
around a safety level (May 11, 2005). Someone asked me about an
environmental clean up. The government told them that the location was
cleaned up to a 90% confidence level of the standard. Would this give the
residents an assurance that everything was safe? I don't have the background
to answer all of this question, but can comment on the Statistical aspects.
Stats: Where is the
confidence interval? (March 31, 2005). A recent letter to the editor,
Child Psychopharmacology, Effect Sizes, and the Big Bang. Mathews M,
Adetunji B, Mathews J, Basil B, George V, Mathews M, Budur K, Abraham S. Am J
Psychiatry 2005: 162(4); 818. complains about an article claiming that a
drug, citalopram, can reduce depressive symptoms A randomized,
placebo-controlled trial of citalopram for the treatment of major depression
in children and adolescents. Wagner KD, Robb AS, Findling RL, Jin J,
Gutierrez MM, Heydorn WE. Am J Psychiatry 2004: 161(6); 1079-83. The letter
writers dispute (among other things) the claim of a statistically and
clinically significant reduction.
Stats: Confidence intervals (November 29,
2004). Dear Professor Mean: Can you give me a simple
explanation of what a confidence interval is?
Stats: Rates versus proportions
(September 15, 2004). Many people use the words "rates" and "proportions"
interchangeably, but there is an important distinction that I draw. A
proportion represents a situation where the numerator and denominator both
represent counts, and the numerator is a subset of the denominator. Rates
represent a situation where the numerator is a count, but the denominator is
in different units (such as the number of patient years of risk) or where the
numerator is not a subset of the denominator (such as number of automobiles
in a town divided by the number of adults living in that town).
Stats: Confidence intervals
for proportions (July 8, 2004). One of the fellows at the hospital asked
me about confidence intervals for proportions. I wrote a couple of simple
spreadsheets to do these calculations. It's important to avoid comparing two
separate confidence intervals to see if they overlap.
Stats: Why 95% confidence limits (May 6, 2002).
Dear Professor Mean:, I've been working with small data sets for some
neuroimaging research that have five (5) treatment and five (5) control
participants. It is not unusual to have such small samples in this kind of
work. My 95% confidence interval (CI) included zero; yet, the 85% confidence
interval did not include zero. I know that the 95% CI is the common one, but
I also know that others are used, but I don't know when to use them.
Therefore, I'd like to know why we use 95% confidence limits all the time?
When is it appropriate to use other CIs and the logic behind making such
decisions?
Stats: Asymmetric confidence intervals
(September 3, 1999). Dear Professor Mean, I found a journal article
with a confidence interval that was asymmetric. For example, the authors
reported a mortality difference of 5% and a 95% confidence interval of -1.2%
to 12%. I can't understand how the CI can be unequally distributed if it uses
the form ESTIMATE +/- 1.96*STANDARD ERROR.
