Risky Business: Framing Statistics

The Gist: Medical literature frequently frames effect size using relative and absolute risks in ways that intentionally alter the appearance of an intervention, an example of framing bias. Effect sizes seem larger using relative risk and smaller using absolute risk [1,2].

Part of our 15 Minutes - 'Stats are the Dark Force?' residency lecture series
Risk from Lauren Westafer on Vimeo.

Perneger and colleagues surveyed physicians and patients about the efficacy of 4 new drugs. They were presented with the following scenarios:
While these numbers reflect the same data, patients and physicians selected that the drug that "reduced mortality by 1/3" was "clearly better" [3].  This indicates both parties are susceptible to the cognitive tricks statistics, particularly the way that relative risk make numbers appear larger and the ways absolute risk makes numbers appear smaller.  Authors can consistently switch between relative and absolute risk to maximize the appearance of benefit and minimize risks, as seen in the following abstract:

Perhap authors should be encouraged to report statistics consistently, using relative OR absolute rather than switching between the two to give the appearance of maximum benefit and minimal risk.

1. Barratt A. Tips for learners of evidence-based medicine: 1. Relative risk reduction, absolute risk reduction and number needed to treat. Canadian Medical Association Journal. 171(4):353-358. 2004
2. Malenka DJ, Baron JA, Johansen S, Wahrenberger JW, Ross JM. The framing effect of relative and absolute risk. Journal of general internal medicine. 1993;8(10):543-8.
3. Perneger TV, Agoritsas T. Doctors and patients' susceptibility to framing bias: a randomized trial. Journal of general internal medicine. 26(12):1411-7. 2011.

P Values: Everything You Know Is Wrong

The Gist:  P values are probably the most “understood” statistic amongst clinicians yet are widely misunderstood.  P values should not be used alone to accept or reject something as “truth” but they may be thought of as representing the strength of evidence against the null hypothesis [1].

Part of our 15 Minutes - 'Stats are the Dark Force?' residency lecture series

P: Everything You Know Is Wrong from Lauren Westafer on Vimeo.

At various times in my life I, like many others, have believed the p value to represent one of the following (none of which are true):
    • Significance
      • Problem:  Significance is a loaded term.  A value of 0.05 has become synonymous with “statistical significance.”  Yet, this value is not magical and was chosen predominantly for convenience [3].  Further, the term “significant” may be confused with clinical importance, something a statistic cannot answer.
    • The probability that the null hypothesis is true.
      • Problem: The calculation of the p value includes the assumption that the null hypothesis is true.  Thus, a calculation that assumes the null hypothesis is true cannot, in fact, tell you that the null hypothesis is false.
    • The probability of getting a Type I Error 
      • Background: Type I Error is the incorrect rejection of a true null hypothesis (i.e. a false positive) and the probability of getting a Type I Error is represented by alpha. Alpha is often set at 0.05 so that there is a 5% chance you are wrong if you reject the null hypothesis. This is a PRE test calculation (set before the experiment)
      • Problem: Again, the calculation of the p value assumes the null hypothesis is true. The p value only tells us the probability of getting the data we did, it does NOT speak to the underlying truth of whatever is being tested (i.e. efficacy). The p value is also a POST test calculation.
      • The error rate associated with various p values varies, depending on the assumptions in the calculations, particularly prevalence. However, it's interesting to look at some of the estimates of false positive error rates often associated with various p values:  p=0.05 - false positive error rate of 23-50%; p=0.01 - false positive error rate of 7-15% [5].
P value is the probability of getting results as extreme or more extreme, assuming the null hypothesis is true. Originally, this statistic was intended to serve as a gauge for researchers to decided whether or not a study was worth investigating further [3].  
  • High P value - data are likely with a true null hypothesis [Weak evidence against the null hypothesis]
  • Low P value - data are UNlikely with a true null hypothesis [Stronger evidence against the null hypothesis]
Example:  A group is interested in evaluating needle decompression of tension pneumothorax and proposes the following:
    • Hypothesis - Longer angiocatheters are more effective than shorter catheters in decompression of tension pneumothorax.
    • Null hypothesis - There is no difference in effective decompression of tension pneumothorax using longer or shorter angiocatheters.
A group, Aho and colleagues, did this study and found a p value of 0.01 with 8 cm catheters compared with 5 cm catheters.  How do we interpret this p value?  
  • We would expect the same number of effective decompressions or more in 1% of cases due to random sampling error.  
  • The data are UNLIKELY with a true null hypothesis and this is decent strength evidence against the null hypothesis.
Limitations of the Letter “P”
    • Reliability.  P values depend on the statistical power of a study. A small study with little statistical power may have a p value greater than 0.05 and a large study may reveal that a trivial effect has statistical significance [2,4].  Thus, even if we are testing the same question, the p value may be "significant" or "nonsignificant" depending on the sample size.
    • P-hacking.  Definition:  "Exploiting –perhaps unconsciously - researcher degrees of freedom until p<.05" Alternatively: "Manipulation of statistics such that the desired outcome assumes "statistical significance", usually for the benefit of the study's sponsors" [7].
      • A recent study of abstracts between 1990-2015 showed 96% contained at least 1 p value < 0.05.  Are we that wildly successful in research? Or, are statistically nonsignificant results published less frequently (probably).  Or, do we try to find something in the data to report as significant, i.e. p-hack (likely).
P values are neither good nor bad. They serve a role that we have distorted and, according to the American Statistical Association: The widespread use of “statistical significance” (generally interpreted as “p ≤ 0.05”) as a license for making a claim of a scientific finding (or implied truth) leads to considerable distortion of the scientific process [1].  In sum, acknowledge what the p value is and is not and, by all means, do not p alone.

  1. Wasserstein RL, Lazar NA. The ASA’s statement on p-values: context, process, and purpose. Am Stat. 2016;1305(April):00–00. doi:10.1080/00031305.2016.1154108.
  2. Goodman S. (2008) A dirty dozen: twelve p-value misconceptions. Seminars in hematology, 45(3), 135-40. PMID: 18582619 
  3. Fisher RA. Statistical Methods for Research Workers. Edinburgh, United Kingdom: Oliver&Boyd; 1925.
  4. Sainani KL. Putting P values in perspective. PM R. 2009;1(9):873–7. doi:10.1016/j.pmrj.2009.07.003.
  5. Sellke T, Bayarri MJ, Berger JO.  Calibration of p Values for Testing Precise Null Hypotheses.  The American Statistician, February 2001, Vol. 55, No. 1
  6. Chavalarias D, Wallach JD, Li AHT, Ioannidis JPA. Evolution of Reporting P Values in the Biomedical Literature, 1990-2015. Jama. 2016;315(11):1141. doi:10.1001/jama.2016.1952.
  7. PProf.  "P-Hacking."  Urban Dictionary. Accessed May 1, 2016.  Available at: http://www.urbandictionary.com/define.php?term=p-hacking

Percutaneous Chest Tubes: The Humane Choice

The Gist:  Small bore percutaneous catheters, often referred to as "pigtail" catheters, should be the initial means of treating many pneumothoraces and select other drainable thoracic pathologies as they cause less pain and capitalize on the commonly used seldinger technique [1-10].

Traditional tube thoracostomy is an invasive procedure.  For the past several years, international guidelines, individuals in the Free Open Access Medical education (FOAM) community, and various institutions have moved towards placing more pigtail catheters for urgent thoracic pathology.  Yet, this practice is still not ubiquitous.  I recently gave a talk on this to my program and, in the spirit of FOAM, have shared it:

Technique - Watch this video by Dr. Larry Mellick 
  • Seldinger style: uses a technique with which we are intimately familiar. The majority of emergency providers have likely done far more central lines than open tube thoracostomies. As such, a technique mentally and mechanically familiar to providers may be preferable.
  • Pearls for placement - Kulvatunyou and colleagues suggest "POW" pearls for placement.
    • P -Perpendicular: Ensure the finder needle is perpendicular to the rib during placement
    • O -Over the rib: Like chest tubes, pigtails go over the rib to avoid injury to the neurovascular bundle
    • W -Wary of wire kinking:  The wire may be prone to kinking, particularly upon dilation through the tough intercostal muscles.
  • Pneumothorax
    • Spontaneous pneumothorax: The British Thoracic Society has recommended small bore tubes over traditional chest tubes since 2010.
    • Traumatic pneumothorax:  Use of pigtail catheters have increased in many trauma communities, with success rates comparable to large bore chest tubes [8-11].
  • Effusions - pigtail catheters are frequently used to drain effusions, particularly simple effusions. Most of the primary literature on this topic has been conducted in children with parapneumonic effusions and has demonstrated that this technique is successful and safe [13].
  • Hemothorax/Complex fluid - Larger bore tubes (28F and larger) are typically used to drain hemothorax due to the feared complication of retained hemothorax.  A prospective review of 36, 14F pigtail catheters placed for hemothorax in trauma patients found no significant differences in complications or success between pigtails or chest tubes but wasn't powered to find important, infrequent complications [11].  An animal study found
The Good:
  • Less Painful - In addition to the procedure not requiring large, forceful separation of the and unsurprisingly, placing a smaller tube in the chest causes less pain, even 2 days after the procedure [10].
    • Pain in pigtail vs chest tube patients: Day 0 3.2  vs 7.7; p<0.001, Day 1 1.9 vs 6.2; p<0.001, Day 2 2.1 vs 5.5; p=0.04 (note: no power calculation performed)
  • Easy/Familiar Procedure - as above under "Seldinger technique"
  • May reduces some complications - The literature suggests that complications are typically at least equivalent between larger chest tubes and pigtails. More serious complications are difficult to quantify given the infrequency.
    • One study did show that infections were reduced in the pigtail group, possibly due to technique or a larger nidus for infection [2].
  • Outpatient treatment possible - In select patient groups with spontaneous pneumothorax and excellent follow up, a pigtail catheter may be connected to a heimlich valve and the patient may be discharged [7].
The Bad:
  • More predisposed to kinking - Due to the small, flexible tubing, these tubes may kink and obstruct the lumen.  The trauma literature suggests these complications may occur in 2-8% of cases [8-10].
  • Clogging - Drainage of some complex fluids (loculated effusion/hemothorax) may be more problematic through pigtail catheters as the small lumen may be easier obstructed with clot.
  • Time? The belief exists that open thoracostomy more expediently relieves pneumothorax compared with the percutaneous technique and is thus preferred in emergent, life-threatening situations. To date, there's no literature to support or refute this and the time a tube takes is likely provider dependent.
  • It's less cool - A certain pride and thrill exists with performing invasive procedures.  In discussions with individuals regarding barriers to uptake of the percutaneous technique the theme arose that performing this technique would demonstrate some sort of weakness by the provider. Note: this notion is not supported or addressed by the literature and is merely a thought about subconscious provider bias
1. Laws D et al. BTS guidelines for the insertion of a chest drain. Thorax. 2003 May;58 Suppl 2:ii53-9.
2.  Benton IJ, Benfield GF. Comparison of a large and small-calibre tube drain for managing spontaneous pneumothoraces. Respir Med. 2009 Oct;103(10):1436-40.
3. Dull KE, Fleisher GR. Pigtail catheters versus large-bore chest tubes for pneumothoraces in children treated in the emergency department. Pediatr Emerg Care. 2002 Aug;18(4):265-7.
4. Gammie JS et al. The pigtail catheters for pleural drainages: a less invasive alternative to tube thoracostomy. JSLS. 1999 Jan-Mar;3(1):57–61.
5. Kuo HC, et al. Small-bore pigtail catheters for the treatment of primary spontaneous pneumothorax in young adolescents. Emerg Med J. 2013 Mar;30(3):e17.
6.  Repanshek ZD, Ufberg JW, Vilke GM, Chan TC, Harrigan RA. Alternative Treatments of Pneumothorax. J Emerg Med. 2013 Feb;44(2):457-466.
7. Hassani B, Foote J, Borgundvaag B. Outpatient management of primary spontaneous pneumothorax in the emergency department of a community hospital using a small-bore catheter and a Heimlich valve. Acad Emerg Med. 2009 Jun;16(6):513-8.
8. Kulvatunyou N, Vijayasekaran A, Hansen A, et al. Two-year experience of using pigtail catheters to treat traumatic pneumothorax: a changing trend. J Trauma. 2011 Nov;71(5):1104-7.
9. Rivera L, O’Reilly EB, Sise MJ, et al. Small catheter tube thoracostomy: effective in managing chest trauma in stable patients. J Trauma. 2009 Feb;66(2):393–9
10.  Kulvatunyou N, et al. A prospective randomized study of 14-French pigtail catheters vs 28F chest tubes in patients with traumatic pneumothorax: impact on tube-site pain and failure rate. EAST Annual Surgical Assembly, Oral paper 12, Jan 17, 2013.
11. Kulvatunyou N, Joseph B, Friese RS, et al. 14 French pigtail catheters placed by surgeons to drain blood on trauma patients. J Trauma Acute Care Surg. 2012;73(6):1423–1427. 
12. Russo RM, Zakaluzny SA, Neff LP, et al. A pilot study of chest tube versus pigtail catheter drainage of acute hemothorax in swine. J Trauma Acute Care Surg. 2015;79(6):1038–1043. 
13.  Liu YH, et al. Ultrasound-guided pigtail catheters for drainage of various pleural diseases. Am J Emerg Med. 2010 Oct;28(8):915-21
14. Inaba K, Lustenberger T, Recinos G. Does size matter? A prospective analysis of 28-32 versus 36-40 French chest tube size in trauma. The journal of trauma and acute care surgery. 72(2):422-7. 2012.