Reviewer\'s quick guide to common statistical errors in scientific papers
Reviewer's quick guide to common statistical errors in scientific papers
Design errors
Sample size for human subjects
Many studies are too small to detect
even large effects (Table 1).
Table 1: Guide to sample size
Expected
difference
(p1-p2)
Tota...
Reviewer's quick guide to common statistical errors in scientific papers
Design errors
Sample size for human subjects
Many studies are too small to detect
even large effects (Table 1).
Table 1: Guide to sample size
Expected
difference
(p1-p2)
Total sample
size required*
5% 1450-3200
10% 440-820
20% 140-210
30% 80-100
40% 50-60
* 5% significance level, 80% power. Smaller numbers
may be justified for rare outcomes (p1 <.1)
Look for:
• Clinical trials should always report
sample size calculations
• Authors with 'negative' results (i.e.
found no difference) should not
report equivalence unless
sufficiently powered -"absence of
evidence is not evidence of
absence"
Bias
Randomisation is the best way of
avoiding bias but it is not always possible
or appropriate.
Some biases affecting observational
studies:
Treatment-by-indication bias: different
treatments are given to different groups
of patients because of differences in their
clinical condition.
Historical controls: will tend to
exaggerate treatment effect as recent
patients benefit from improvements in
health care over time and special
attention as a study participant. Recent
patients are also likely to be more
restrictively selected.
Retrospective data collection: availability
and recording of events and patient
characteristics may be related to the
groups being compared.
Ecological fallacy: an association
observed between variables on an
aggregate level does not necessarily
represent the association that exists at
the individual level.
Some biases affecting observational
studies and clinical trials:
Selection bias: low response rate or high
refusal rate – were patients that
participated different to those that did
not?
Informative dropout – was follow-up
curtailed for reasons connected to the
primary outcome? If so, imbalance in
dropout rates between the groups being
compared will introduce bias.
Bias in clinical trials:
No-one should know what the next
random allocation is going to be as this
may affect whether or when the patient is
entered into the trial. Using date of birth,
hospital number, or simply
alternating between treatments
is therefore inappropriate.
Central randomisation is ideal.
Unblinded assessment of
outcomes may be influenced by
knowledge of the treatment
group.
Look for:
• Appreciation and
measures taken to reduce
bias through study design
• Selection of patients,
collection of data, definition
and assessment of
outcome and, for clinical
trials, method of
randomisation should be
clearly described
• Number and reasons for
withdrawal should be
reported by treatment
group
• Appropriate analytic
methods such as multiple
regression should be used
to adjust for differences
between groups in
observational studies
• Authors should discuss
likely biases and potential
impact on their results
Method comparison studies
If different methods are
evaluated by different observers
then the method differences are
confounded with observer
differences. The study must be
repeated with each observer
using all methods.
Analysis errors
Failure to use a test for trend on
ordered categories (e.g. age-
group).
Dichotomizing continuous
variables in the analysis
(acceptable for descriptive
purposes).
Using methods for independent
samples on paired or repeated
measures data. An example is
using both arms or legs of the
same patient as if they were two
independent observations.
Using parametric methods (e.g.
t-test, ANOVA or linear
regression) when the outcome
or residuals have not been
verified as normally distributed.
Over using hypothesis tests (P-
values) in preference to
confidence intervals.
One-tailed tests are very rarely
appropriate.
Failing to analyse clinical trials
by intention-to-treat.
Obscure statistical tests should be
justified and referenced.
Comparing P-values between subgroups
instead of carrying out tests of interaction
is incorrect. Some may wrongly conclude
from these results that:
P>0.05
P<0.05Subgroup A
Subgroup B
1
Treatment effect with 95% CI
the subgroup affects response to
treatment, based on comparing P-values.
A test of interaction would show no
evidence of any effect of the grouping on
response.
Correlating time series: any two variables
that consistently rise, fall or remain
constant over time will be correlated.
'Detrended' series should be compared
instead.
Method comparison studies
Correlation ≠ agreement
Perfect agreement
Perfect correlation
M
e
th
o
d
B
Method A
Higher correlation can be induced by
including patients with extreme
measurements. Limits of agreement
should be calculated according to
method of Bland and Altman. Adequate
agreement between methods is a clinical
not a statistical judgement.
Multiple testing
Conclusions should only be drawn from
appropriate analyses of a small number
of clear, pre-defined hypotheses. Results
from post-hoc subgroup or risk-factor
analyses should be treated as
speculative. If many such tests have
been carried out adjustment for multiple
testing should be considered.
Comparing groups at multiple time points
should be avoided – a summary statistics
approach or more complex statistical
methods should be used instead.
Further reading:
CONSORT: http://www.consort-statement.org
Greenhalgh T. How to read a paper: Statistics for the
non-statistician. I: Different types of data need different
statistical tests. BMJ 1997;315:364-366
Bland JM, Altman DG. Statistical methods for
assessing agreement between two methods of clinical
measurement. Lancet 1986;1:307-310. Available online
at http://www-users.york.ac.uk/~mb55/meas/ba.htm
BMJ Statistics Notes: http://www-
users.york.ac.uk/~mb55/pubs/pbstnote.htm
Produced by Tony Brady
Sealed Envelope Ltd
http://www.sealedenvelope.com
本文档为【Reviewer\'s quick guide to common statistical errors in scientific papers】,请使用软件OFFICE或WPS软件打开。作品中的文字与图均可以修改和编辑,
图片更改请在作品中右键图片并更换,文字修改请直接点击文字进行修改,也可以新增和删除文档中的内容。
该文档来自用户分享,如有侵权行为请发邮件ishare@vip.sina.com联系网站客服,我们会及时删除。
[版权声明] 本站所有资料为用户分享产生,若发现您的权利被侵害,请联系客服邮件isharekefu@iask.cn,我们尽快处理。
本作品所展示的图片、画像、字体、音乐的版权可能需版权方额外授权,请谨慎使用。
网站提供的党政主题相关内容(国旗、国徽、党徽..)目的在于配合国家政策宣传,仅限个人学习分享使用,禁止用于任何广告和商用目的。