Common sense statistics

30-05-2020

Raja Selvaraj

One of the best tools in the statistical detective’s toolkit is common sense

Sainani, K. L. (2019). How to Be a Statistical Detective. PM&R. 10.1002/pmrj.12305

Statistics: A subject which most statisticians find difficult but in which nearly all physicians are expert.

Anon

- 45 year old male
- Chest pain since 4 hours
- 2 mm ST elevation in anterior leads
- Raised Trop T

- Large study - 17,187 patients
- Acute MI < 24 hours
- 2 x 2 design
- Streptokinase vs placebo infusion and Aspirin vs placebo tablets
- Significant separate benefit, benefit also additive

ISIS-2 collaborative group. Randomised Trial of Intravenous Streptokinase, Oral Aspirin, Both, or Neither Among 17,187 Cases of Suspected Acute Myocardial Infarction: ISIS-2. Lancet. 1988 Aug 13;2(8607):349-60.

- Date of birth - October 01
- Does it change treatment ?

"All these subgroup analyses should, perhaps, be taken less as evidence about who benefits than as evidence that such analyses are potentially misleading."

- Multiple comparisons / multiple testing
- p-hacking
- publishing bias

Thresholds for significance based on one comparison or test. Thus 5% chance of the finding being random gives 95% confidence its real

- What is the chance that the difference we are seeing is due to purely random variation ?

- 400 patient attenders in Cardiology OPD
- BP measured
- 50 found to have systolic BP > 130 mm Hg
- Advised to drink one extra glass of water each day for one week
- Significant fall in BP after one week !

- In any series with complex phenomena that are dependent on many variables, where chance is involved, extreme outcomes tend to be followed by more moderate ones.
- Confounder in many trials - eg. Renal artery denervation

- Ratio of PA diastolic pressure / RA pressure predicts increase in MVA during BMV
- What question do you want to ask ?

Anscombe, Francis J. (1973) Graphs in statistical analysis. American Statistician, 27, 17–21.

Sainani, K.L. (2020), How to Be a Statistical Detective. Journal of Injury, Function and Rehabilitation, 12: 211-215. 10.1002/pmrj.12305

- A cure is prepared according to homeopathic principles
- In a study the adverse outcomes are reduced compared to controls
- Significantly lesser, p = 0.03 !

- Subgroup analysis
- Multiple testing and comparisons
- Regression towards the mean
- Placebo effect
- Bayesian and frequentist approaches

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