statistics you can use: practical use of statistics in reading medical research literature
Post on 01-Jan-2016
37 Views
Preview:
DESCRIPTION
TRANSCRIPT
Statistics you can use:Practical use of statistics in reading medical research literaturePAS 610
June 21, 2005
Robert D. Hadley
PhD, PA-C
The basics:
““There are three kinds of lies: lies, damned There are three kinds of lies: lies, damned lies, and statistics.”lies, and statistics.”
Benjamin Disraeli, British politician (1804 - 1881)
The value of statistics:
Four economists are going to a meeting on the same train as four statisticians. The economists can't help noticing that the statisticians only buy a single ticket, where they bought four. When they inquire, the statisticians say, "Don't worry, you'll see."
They get on the train, and when the conductor starts in their car the four statisticians all lock themselves in the WC. When the conductor knocks on the WC door and yells "TICKET", they slide the ticket out under the door, and the conductor stamps it and slides it back. After he's gone, the statisticians emerge.
At the station on the way back from the meeting, the economists buy only one ticket, but they can't help noticing that the statisticians don't buy any. When they inquire, the statisticians say, "Don't worry, you'll see.“
As the conductor approaches their car, the economists all pile in the nearest WC and lock the door. One of the statisticians goes and knocks on the door; the economists slide the ticket out. The statisticians take the ticket and lock themselves in the WC at the other end of the car, repeating their maneuver of the previous trip. The economists get thrown off the train.
Moral: Don't use statistical methods you don't understand.
“Practical” vs. STA 570
Ways to represent dataSample vs. population
Ways to compare data e.g. Chi-square, Student’s t-test, ANOVA/
ANCOVA, Odds ratios and CI, Cox proportional hazard model, Spearman ranked correlation coefficients, multivariate regression analysis
Appropriateness of test for the way data were collected
Terms: basics
Mean Median
Quartiles, tertiles, etc. Mode Rank Nominal Ordinal Population Sample Variance Standard deviation
Normal distribution Z-scores, T-scores Correlation Parametric vs.
Nonparametric Hypothesis testing
1- vs. 2-tailed Significance levels Confidence intervals Statistical power
Terms: medical literature-specific Intention to treat Kaplan-Meier curves ROC curves Meta-analysis representations Odds ratios/Relative Risk Risk reduction Number needed to treat
Over what time period? For what outcome?
Number needed to harm
Concepts
Descriptive vs. inferential statistics Type I and II errors
DescriptiveDescriptive StatisticsStatistics DescriptiveDescriptive StatisticsStatistics
InferentialInferential StatisticsStatistics InferentialInferential StatisticsStatistics
IncludesIncludes Collecting Organizing Summarizing Presenting data
IncludesIncludes Making inferences Hypothesis testing Determining relationships Making predictions
Inferential errors
Type I (alpha)
Incorrectly reject the null hypothesis
Infer that something is significant when it is not
Type II (beta)
Incorrectly accept the null hypothesis
Infer that something is not significant when it really is
So, which is better to do?
Which way does “intention to treat” skew the inference?
Study design
Ask the right question in the right way
Statistical power
Choose the appropriate sample size
Standard deviation and Z-scores
Note: “normal” range for lab tests is ± 2 s.d.
Z and T scores in medicine
Bone density data are reported as T-scores and Z-scores. T-scores represent the number of SDs from the normal young adult mean bone density values, whereas Z-scores represent the number of SDs from the normal mean value for age- and sex-matched control subjects.
Results showing Z-scores of −2.0 or lower may suggest a secondary cause of osteoporosis.
Osteoporosis drug treatment
Data Representation
Relative risk, odds ratios, likelihood ratios, hazard ratios
Odds ratios in meta-analyses
Relative risk
What do unequal CI bars mean?
Meta-analyses
“Gold standard” is randomized, placebo-controlled, multi-center, double blind clinical trial
“Platinum standard” is a meta-analysis of multiple “gold standard” trials by different investigators addressing the same question (rarely available)
Can make use of small studies that by themselves do not achieve statistical significance
Meta-analyses
How it’s done: Search on a specific topic Use predefined inclusion/exclusion criteria
for studies that relate to topic• e.g. must be RCT, must measure same specific
outcome (like cardiovascular events), etc. Combine all studies that meet criteria Use statistics appropriate to the way data
were gathered in the included studies Arrive at a conclusion that was impossible
with the individual studies that were included
Other anti-platelet drug (Reg. 1)
Aspirin (Reg. 2)
Antiplatelet therapy for CVD
BMJBMJ 2002; 324:71-86 2002; 324:71-86
Data Representation
Kaplan-Meier survivorship, and cumulative incidence of events
Both are a cumulative measure of something happening
Kaplan-Meier
Bortezomib or High-Dose Dexamethasone for Relapsed Multiple Myeloma
N Engl J Med 2005;352:2487-98
Use of quintiles to choose cutoff points
0
1
2
3
4
Years
Cu
mu
lati
ve i
nci
den
ce (
%)
ASCOT-LLA: Trial Stopped Nearly 2 Years Early
Sever PS et al. Lancet. 2003;361:1149-1158.
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
36% RRR nonfatal MI + fatal CHD(P =.0005)
Atorvastatin 10 mg No. of events: 100Placebo (diet and exercise only) No. of events: 154
All patients counseled ondiet and exercise
What is approximate NNT for 1 year?
Data Representation
2x2, PPV, Chi-Square ROC curves
ROC
Receiver operator characteristic curvesRadar operators’ ability to distinguish
signal from noise Higher area under curve (AUC),
higher reliability for a given test Plot true positives vs. false positives
ROC
ROC value: 0.65 (0.61-0.70)
Data Representation
Correlationmany statistical methods
Correlation of clinical data
Correlation of clinical data
Is r=0.16 a strong correlation?
Can we conclude that CRP and LDL are related?
Box plots (not common)
25th percentile, median and 75th percentile indicated in each box
Other interesting data representation
Neater than a true scatter plot
Simple to interpret
Nissen et al, N Engl J Med 2005;352:29-38
An example:
Peterson RC, Thomas RG, Grandman M, Bennet D, Doody R, Ferris S, et al. Vitamin E and Donepezil for the Treatment of Mild Cognitive Impairment. N Engl J Med 2005;352:2379-88.
Available at: http://content.nejm.org/cgi/content/full/352/23/2379
Questions:
What kind of study is this? How large is the study? What are the inclusion/exclusion criteria? What is the outcome measured? What is the intervention? What are the statistical tests, and are they
appropriate? What data representations are used? Is the result statistically significant? Is the result clinically significant? How does this knowledge affect my practice?
top related